I suppose it could be done that way. But that seems like a lot of extra work, and you also lose the compilers help. Also, in my experience changes like this actually do involve lots of subtly broken code - think about it, anyone using fold on a network error is making a mistake (retrying the use of a closed connection).

If you fix the behavior everyplace the compiler complains, you are guaranteed to resolve most issues (ignoring overloaded methods, e.g. getOrElse). If you use a toOption method on the union type to make a tiny atomic change, the compiler will no longer complain in all the places that mistakes are being made.

I suppose an intermediate way might be to rename f to f_DEPRECATED, replace everywhere, and build the new method returning the right type. But in my experience its better just to bite the bullet and change everything.