[thingification]

If you can instead construct a list of existing instances to grandfather in, that doesn't suffer from this problem. Of course many linting tools do this via "ignore" code comments.

That feels less arbitrary than a magic number (because it is!) and I've seen it work.

[Etheryte]

We used this approach to great effect when we migrated a huge legacy project from Javascript to Typescript. It gives you enough flexibility in the in between stages so you're not forced to change weird code you don't know right away, while enforcing enough of a structure to eventually make it out alive in the end.