You can administer the site using CS3 and it won't mess up
your Contribute 3 users. If you have any Contribute 3
administrators, though, they will be locked out of the admin
features once you administrate the site with CS3. There are "fixes"
for this, but they didn't work for us. We decided to just keep all
administrators on 3.1 until all were ready to upgrade to CS3.
(Replace the _mm file with a backup to restore access to Contribute
3 admins and then don't use CS3 to administrate again.) You can
also have some users who use CS3 and some with Contribute 3 without
issue (at least none we've found so far).