Support evolving to a subclass (possibly any class)

Recently I wrote a modification to the attrs.evolve function that works like this:

@attrs.define
class Foo:
  a: int

@attrs.define
class Bar(Foo):
  b: str

foo = Foo(1)
bar = evolve_as(foo, Bar, b="2")
print(bar)
# Bar(a=1, b='2')

This was very useful for my particular use-case, which involved enriching a data object in a pipeline, so I thought that this might be a good fit for attrs.

My implementation did not require Bar to be a subclass of Foo, but I'm not sure about the design implications of that. Might be better to limit this to subclasses only.

Start by reading the existing attrs.evolve implementation and its surrounding public API behavior. Clarify whether evolving to unrelated classes is supported or only subclass targets, then define the expected result and compatibility behavior before identifying the tests needed to confirm the chosen design.