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Plan and prioritise · Add-on

Plan a move off a legacy system

Replacing an old system all at once is risky. Move piece by piece, delivering value along the way.

About 5-15 daysEvergreenLast reviewed 5 November 2026

Who this applies to

Skills it builds
Technical and AI fluencyDelivery and execution
PM types
Transformation
Product types
All
Starting points
All
Ways of working
All
Experience
All
Size of organisation
All

Steps, by experience

Beginner (under 1 year)

  1. List what the old system does, including hidden jobs nobody mentions.
  2. Find out who depends on it and what data it holds.
  3. Split the move into small pieces you can switch over one at a time.
  4. Start with a piece that is valuable and low risk.
  5. Plan how data will move and how you will switch back if needed.
  6. Agree when the old system will be switched off.

Intermediate (1 to 4 years)

  1. Inventory functions, users and data.
  2. Plan incremental migration slices.
  3. Plan data migration and rollback.

Experienced (5+ years)

  1. Use an incremental replacement pattern rather than a big-bang rewrite.
  2. Deliver new value alongside migration to keep support.
  3. Manage the cost of running both systems.

Who can do this

Owner: Product manager

Can be delegated to

  • Architect or Tech Lead: Design the migration approach
  • Business Analyst: Inventory functions and hidden jobs

Reviewed by: Architect, Tech Lead

Tool examples

Miro
Map legacy functions and plan which moves first.
Jira
Track each migration slice as an epic.

Definition of done

  • Functions, users and data inventoried
  • Incremental plan agreed
  • Rollback and switch-off plans agreed

Common pitfalls

  • Big-bang replacement
  • Missing hidden functions
  • Never switching the old system off

If you lead the team

What good looks like: Legacy systems are replaced piece by piece, delivering value along the way.

Ask your PM

  • What's the next piece we can move, and what value does it bring?
  • How do we keep old and new systems in step?

Warning signs

  • The plan is one big switch-over date.
  • Value only arrives at the very end.

Do these first

Sources