I'm sure you are are that if we make it very easy for people to upgrade
from an arbitrary point in the release cycle, to another arbitrary point,
the failure mores increase.

A good example is the current tech@ thread "Re: Unprivileged font cache: src",
please see my most recent reply to that.

But there have been other circumstances like this.  You are exposing admins
to something which was not tested.  We've only ever really tested
   R-.1 -> R
   R-.2 -> R
   R-.1 -> S
   R-.2 -> S
   S (less than 6 months old) -> S
   S (less than a year old) -> R

I won't argue jumping around is useful to find bugs.

That's why we carefully documented it this way:

     -R version
             Upgrade to a specific release version.  Only upgrades from one
             version to the next are tested.  Skipping versions may work.
             Downgrading is unlikely to work.

But surely your diff needs a change to this wording.  That archive
directory does not contain releases, it contains snapshot dates.

I think this needs a bit more clarity.