You can override the built-ins by re-defining the symbol in any of: local functions, enclosing functions, or the global scope and it will impact the entirety of that scope.

If I'm understanding your comment correctly this time, it would look something like:

getattr(thing, b'attribute_name')

getattr(thing, u'attribute_with_unicode_characters')

This would work in both python 2.7 and 3.9 (probably works with other 3.X versions but it's the one I actually tried).

    str = int

Well python is a duck-typed language, so it will handle the unicode string as if it was a bytestring, and this may or may not work correctly. It's up to the modern code to interface correctly with the legacy code. This could mean manually converting between bytestring and unicode string.

Upgrading to a new major version should never require changes to your code if you fixed all deprecation warnings in the previous major version. Otherwise, you make incremental upgrades impossible.

The new alternative should always be introduced as opt-in with the old default deprecated, and then made default in the next major version.