In practice I think the handful of packages that this happens on isn't particularly meaningful or valuable, and in my experience what happens when Debian finds a new key signing packages is.. highly variable. I've seen them just disable signatures when a new key shows up (on major packages even), I've seen them just blanket copy whatever the new key is, I've seen them look at release notes for what the new key is. In one or two cases I've seen them actually track down the project and ask for verification of the new key.
To me, the PGP support in Debian's uscan feels more like security theater than actual security controls, given my experience with the varied responses to a new release being made by a different key.