Is it possible to freeze or peg repositories at a particular version, or to prevent automatic repository version upgrades?  Is it possible to "downgrade" a repository?

### Please describe the problem.

We have a number of repositories on a shared file server.  These repositories are accessed by multiple machines.  Some of these repositories appear to have gotten upgraded and are now unusable on machines running older versions of git-annex.

We're getting this message:
[[!format sh """
user@system:/path/to/repository$ git annex status
git-annex: Repository version 5 is not supported. Upgrade git-annex.
"""]]

The machine experiencing the problem is running Debian Wheezy (Stable).
[[!format sh """
user@system:/path/to/repository$ git version
git version 1.7.10.4
user@system:/path/to/repository$ git annex version
git-annex version: 3.20120629
local repository version: 5
default repository version: 3
supported repository versions: 3
upgrade supported from repository versions: 0 1 2
"""]]

I'm guessing that one of the machines with access to this repository was running a newer version of git-annex, and that the repository was upgraded in the course of some action.

> I took this onboard with v6; upgrade to it is currently optional and will
> only become the default once enough years have passed that it's
> reasonable to assume most people have git-annex 6.x installed.
> 
> I think at this point it's reasonable to assume most people have
> git-annex 5.x installed, so there's no point in trying to change to v3 to
> v5 forced upgrade at this point. So, [[done]] --[[Joey]]
