When to build a migration?

Question

I am building a visual studio like application: User can create a new project, edit it, save it to disk and load. The project isn't exactly like VS it is domain specific.

Like with VS I need a way to migrate project from V1 to V2 as they have a different structure on disk.

When should I build such migration tool?

option 1: Near the end of a release declare "freeze" on format breaking changes and build a converter. cons: During the whole development period developers cannot open a real customer project in the development version since there is no migration path. It can be useful to see how a real project performance / UI looks now.

option 2: At the moment a developer makes a breaking change he should add it to the migration project. Cons: A lot of overhead for dev.

Answer 1

Most likely option 2. Having "real" data is very valuable.

Further more it is much easier to build each conversion when you have the relevant stuff fresh in mind, and you are less likely to forget conversion of some parts of the format.