Migrate Your Custom Softwarefor modern architecture and scalability

Custom software usually has no accurate documentation. The rules are in the code, in the database and in the habits of the staff who use it. Some look like bugs and turn out to be how the business has worked for a decade. So the first deliverable of a custom migration isn't code. It is a written account of what the system actually does, checked with the people who use it.

Replace it in steps when the system can be split: new code handles one area at a time while the old system keeps the rest. Risk stays small and value arrives early, but you run two systems for a while and something has to keep their data consistent.

Rebuild alongside it when the old code can't be separated, as is common with old PHP that has no framework. Both versions run in parallel and you compare their results. This takes more patience, and it is the only way to make a rewrite verifiable.

Leave it alone when the system works, is secure enough and isn't holding anything back. That is an unusual recommendation for a supplier to make, but not every old system is a problem. Sometimes putting it behind a modern API is enough.

A working system plus the handover documentation that the old one never had: what it does, how it is deployed and why the unusual rules exist. That documentation matters as much as the code, because otherwise the next team finds itself in the same position you are in now.