existing installs come with the core16 snap, it may make sense to figure out if core16 needs removing and replacing with core18
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.
neon-settings will now attempt to remove the kf5 snap when it is found to be unused as previously mentioned. the core18 snap will be installed on-demand should the user opt to install one of the kde snaps that use it.