If you want type-safe code you will usually have to rewrite some parts of your app, or most of it if someone butchered the initial implementation.

In theory maybe you are right. In practice it never just adding types. E.g. you need to add null checks, rewrite old parts to follow same style (in case there are old parts in the code) and etc.

This usually happens anyway when you figure out how to do something better. Then need to apply it everywhere else.