Legacy System Modernisation Assumptions Log
This is a practical guide to building a assumptions log for a legacy system modernisation project — a register of the assumptions the plan depends on, to be validated, adapted to the realities of modernising or replacing ageing core systems.
What a Assumptions Log is
A assumptions log is a register of the assumptions the plan depends on, to be validated. For the full concept and how it works in general, see Project Assumption. On a legacy system modernisation project it plays the same role, tuned to this kind of work.
Why it matters for a Legacy System Modernisation project
Legacy System Modernisation projects live or die on modernising or replacing ageing core systems. A well-built assumptions log gives the team a shared, explicit reference for exactly that — reducing ambiguity, aligning stakeholders, and making problems visible early enough to act. Skipping it, or doing it generically, is how legacy system modernisation projects drift into avoidable delay and cost.
What to include
- Assumption
- Impact if wrong
- Owner
- Validation status
Legacy System Modernisation-specific considerations
Tailor the assumptions log to the risks that most often derail legacy system modernisation projects:
- Undocumented legacy behaviour
- Phased cutover risk
- Knowledge loss
Example
On a real legacy system modernisation project, the assumptions log would be shaped by modernising or replacing ageing core systems. In particular, it should explicitly account for the project’s biggest risks — undocumented legacy behaviour, phased cutover risk, knowledge loss — rather than treating them as afterthoughts.