Redmine

Any suggestions?

You should better post to "Help forum" for that topic.

It depends on multiple factors:
- how did you get the redmine sources (svn, git, hg, zip-download)?
- what do you mean by unstable 2.6 - current development state of redmine?
- have you done some changes to the sources?
- have you installed plugins?

If you have no plugins and have made no changes to sources, than it could be as simple as svn switch https://svn.redmine.org/redmine/branches/2.6-stable or git checkout 2.6-stable in the redmine installation directory.

I'm normally switching my installations by checking out the needed branch into another directory and rsync-ing from the old path and inspecting / reverting the changes (by migrating from svn stable branch to the custom git branch based on the same svn stable one).

I would recommend to use git or hg to track local changes (needs some DSCM experience), if you do a little more, than config/database.yml editing.

Yes, but one of migrations (move mail column from users) is irreversible.

Here is the answer: #19233. Apply this patch and rollback to latest migration in stable version. Profit! :)

Also, destroy Setting of your plugins, otherwise you'll get undefined Class/Module parameters error