Updated · Originally published
TL;DR
Every organization that has operated for more than a decade has legacy systems. Managing those legacy systems during ERP modernization is one of the first practical challenges that comes up. The answer is rarely simple — and it is almost never the same for every system in the environment. Here is how to approach it.
In This Article
- Not everything gets migrated and that is fine
- The legacy data problem
- Legacy customizations are a separate conversation
- What good legacy system planning looks like during ERP modernization
- Common questions about legacy systems and ERP modernization
Every organization that has operated for more than a decade has legacy systems. Managing those legacy systems during ERP modernization is one of the first practical challenges that comes up when the decision is made to move to . Some are aging ERP platforms like Dynamics GP or NAV. Others are standalone tools that got bolted on over the years — a spreadsheet-based reporting process here, a homegrown database there, a third-party payroll system that never quite integrated cleanly with anything else. The answer to what to do with them is rarely simple and it is almost never the same for every system in the environment.
When the decision is made to modernize to Business Central, the question of what to do with those legacy systems is one of the first practical challenges that comes up. The answer is rarely simple and it is almost never the same for every system in the environment.
Still on GP or NAV? Legacy system complexity is one of the most common reasons ERP migrations run over schedule. Starting your assessment early — before the implementation partner is engaged — gives you the most control over the outcome.
Not Everything Gets Migrated and That Is Fine
The first instinct many organizations have is to migrate everything. Move all the data, replace all the systems, cut over completely on day one. In practice that approach creates more risk than it manages.
A more effective strategy is to categorize legacy systems before the project starts:
Systems that get replaced by Business Central. Most of what GP or NAV handles — financials, purchasing, inventory, basic reporting — has a direct equivalent in Business Central. These systems retire when Business Central goes live.
Systems that integrate with Business Central. Some third-party tools stay in place but connect to the new ERP through APIs or native connectors. Payroll systems, EDI platforms, eCommerce integrations and CRM tools often fall into this category. The integration needs to be designed and tested as part of the implementation project, not treated as an afterthought.
Systems that get archived. Historical data from legacy systems does not always need to live in Business Central. In many cases it makes more sense to archive it in a read-only format — a SQL database, a reporting environment or even a structured file archive — and migrate only the active records the business needs from day one. This keeps Business Central clean and reduces implementation complexity significantly.
Systems that stay temporarily in parallel. For one or two accounting periods after go-live, some organizations run both systems simultaneously. This is particularly common for finance teams that want to validate that Business Central produces matching results before fully closing out the legacy environment. It adds short-term complexity but reduces risk around financial reporting accuracy.
The Legacy Data Problem
Legacy data is consistently the most underestimated challenge in any ERP modernization project. The volume of data is rarely the issue. The issue is quality.
Records that were adequate in GP or NAV — item masters with inconsistent unit of measure settings, customer records with duplicate entries, open transactions with missing cost layers — often fail validation checks when moved into Business Central. The problems were always there. The migration just makes them visible.
Data cleanup needs to start before the legacy systems ERP modernization project does. Waiting until data migration begins to discover quality issues is one of the most common reasons ERP projects run over schedule and over budget. A data assessment at the beginning of the project, not the middle, gives the team time to address problems before they become go-live blockers.
Legacy Customizations Are a Separate Conversation
Organizations moving from GP or NAV almost always have customizations — modifications to the base system that were written to address a business requirement the standard product did not handle. Every one of those customizations needs to be evaluated individually before the migration begins.
Some can be retired. Business Central handles a lot of things natively that required custom code in older versions of GP or NAV.
Some need to be rewritten as AL extensions. The customization still makes sense but has to be rebuilt for the cloud architecture.
Some reveal underlying process problems. The business rule the customization was enforcing may no longer be accurate, or the process it supported may have changed. This is a good moment to revisit whether the customization should exist at all.
None of these evaluations should happen mid-project. The answers need to be documented before configuration begins so the project scope reflects what is actually being built.
What Good Legacy System Planning Looks Like During ERP Modernization
The organizations that handle legacy systems well during ERP modernization share a few things in common. They start the conversation early — before the implementation partner is engaged, ideally. They inventory everything: every system, every integration, every customization, every data source the business depends on. They make deliberate decisions about what gets migrated, what gets archived and what stays temporarily in place. And they build the data cleanup work into the project plan rather than treating it as something that will sort itself out.
Technology Management Concepts (TMC) has been helping organizations navigate legacy systems ERP modernization since 1984. That experience includes hundreds of migrations from GP, NAV and other legacy Dynamics platforms to — and the legacy system complexity that comes with them.
Planning an ERP modernization? TMC has run hundreds of migrations from GP, NAV and other legacy Dynamics platforms to Business Central. Let us help you assess your environment before the project begins.
Frequently Asked Questions
Do we need to migrate all our historical data during legacy systems ERP modernization?
No and in most cases migrating everything is not recommended. Most organizations migrate active master records and open transactions and archive older historical data separately. A clean starting point in Business Central is worth more than a complete history that adds migration complexity and risk. Your implementation partner should help you define exactly what needs to move and what can be archived.
How long should we run legacy systems in parallel with Business Central?
One to two accounting periods is typical. Running in parallel lets finance teams validate that Business Central produces matching results before fully retiring the legacy system. Beyond two periods the additional confidence rarely justifies the cost and complexity of maintaining two live environments.
What happens to GP customizations during a Business Central migration?
Each customization needs individual evaluation. Some can be retired because Business Central handles the requirement natively. Others need to be rewritten as AL extensions for the cloud architecture. A few may no longer be needed at all. This evaluation should happen at the start of the project — not mid-implementation — so the scope accurately reflects what is being built.
How do we handle third-party systems that currently integrate with GP?
Integrations need to be redesigned for Business Central rather than simply reconnected. The APIs and data structures are different. For each third-party system — payroll, EDI, eCommerce, CRM — the integration method, data flow and testing requirements need to be defined as part of the implementation scope. Treating integrations as an afterthought is one of the more common causes of post-go-live problems.
What is the biggest risk in an ERP modernization project?
Data quality. Organizations consistently underestimate how much cleanup legacy data needs before it will import correctly into a modern ERP. The problems are not created by the migration — they were always there. The migration just makes them visible. Starting a data assessment early and building cleanup time into the project plan is the single most effective thing an organization can do to protect its go-live timeline.
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Implementing new systems can definitely be frustrating, but if you are willing to learn them they can often be more efficient. Legacy software I would agree, will make your life a lot easier once you start using it and as you are able to see all the features it offers! The legacy software system can can help in all your business software needs! Thank you for sharing your input on the legacy software and the tips that you have for other customers in using it!