Actually, when you upgrade a Fedora n to n+1, you need to manually import the public PGP key using by packages in the news repositories. This step can be avoided by putting all publics PGP keys used by Fedora project (on all these actives repositories) inside one package available in repo to all version of Fedora. With this, when the repositories of the futur version of Fedora are available, users under stables versions of Fedora only needs to update their systems to have the new PGP key. So, if they want to upgrade to new version of Fedora, they only need to lanch fedup.
This has nothing to do with rpm itself. fedora-release is where all the public keys are currently stored, reassigning... That said, having to importing a new pubkey at the time of distro upgrade doesn't seem like a particularly big inconvenience to me.
We started shipping the next releases gpg keys awhile ago now.