Add the possibility in the decorator to specify the `upgrade` and `downgrade`
functions that convert values between APIs. Both function have the same API:
upgrade(self, value, *args, **kwargs)
downgrade(self, value, *args, **kwargs)
The arguments ``self``, ``*args`` and ``**kwargs`` are the ones passed to the
method, following its new-API signature.
Fixes#4944, #7830.