How does the replacement of an old system proceed?

The Three Approaches

Cutover date. Old system on Friday, new system on Monday. Sounds simple, but it is the riskiest option: All problems occur simultaneously, under time pressure, and there is barely any way back.

Parallel operation. Both systems run in parallel, one area after the other migrates. More resource-intensive, but every step is reversible.

Side-by-side with an intermediate layer. New functions are created in the new system, while the old system remains for the rest. Via an intermediate layer, users notice nothing. Resource-intensive, but often the only viable option for large systems.

What Really Makes the Migration Difficult

Not the technology – the data quality. Data accumulated over years contains duplicates, outdated entries, values in the wrong fields, and special cases that only one person can explain.

This work cannot be delegated to anyone else. You should start it early, not in the last week.

The Sequence That Has Proven Effective

1. Inventory: what exists, what is actually needed
2. Cleaning – in the old system, before migration
3. Test import into a test environment
4. Sample inspection by the department
5. Migration of the first area, live
6. After stabilization: the remaining areas

What No Plan Includes

The period after the migration. Plan for several weeks of increased workload: Questions, minor corrections, acclimatization. Whoever does not plan for this experiences the transition as chaos, even if it proceeds normally.