Similarly to T10105 there is no real value in this.

For one, figuring out if a migration is called for (install new & remove all) is super non trivial since snapd doesn't expose the metadata readily available (i.e. we can't easily check if any installed snap requires the runtime).

As a quick and dirty solution to the problem at large I think it may be viable to write a tiny script that simply drops the old kf5 snap iff there are no other snaps installed. This is the most conservative approach to clean up I feel. We could do a similarly dirty trick by looking at connected interfaces (i.e. snaps that use kf5 need to have a connection to it), but I am not confident that is a reliable or in fact save data point to make the decision on whether to remove the kf5 snap.