It is one that can be found via static analysis. That may not be true if behavior is changed. In some cases, this may be "ok" (see: Go's recent loop changes), but in others, it may not.
Yes, but "Unknown function datetime.utcnow" is much easier to diagnose than somewhere in your application, possibly inside a third-party library or maybe even in a different service altogether (because the change traversed serialization and deserilialization) throwing an exception because you can't compare two datetimes.
But I think changing the behaviour and not the name is still a very bad idea.