Comment # 7 on bug 1205009 from Stefan Hundhammer

So, if we choose to offer such a feature, we need to harden it against casual
misuse.

One possibility might be to put that whole thing into a small sub-workflow:
Offer to "keep the old repos with their old priority and change the URLs to
point to the new release", then do that URL-change in a semi-automatic way
(search and replace the old release, say, "15.4", with the new one, say,
"15.5"), and offer each one for manual editing. Possibly we should also try to
access those URLs to check that the target actually exists.

Just offering a very simplistic "keep the old repos and let me handle it
manually" would be leaving too many users out in the rain IMHO.