Document and test routine from release-to-release
Something concrete we can put in the RELEASES file about this. rgdd's current thinking is that we could perhaps have a VM on release X; snapshot it; and then ensure that we're able to upgrade to X+1. On success we snapshot and repeat next time, for X+1 -> X+2.
And specify concretely what we're testing to consider it "to work".
We can also consider making the VM/snapshots available.