BES 3.6.7 is fine to upgrade to 4.0 and 4.1. As long as you back up the DB you are fine.
It's only 12 users so the chances of you having a nightmare upgrade is pretty rare.
Worst case scenario:
Upgrade goes bad and you install BES 4.1.5 fresh and activate the users anew.
Again, back up the DB and and run the installer inplace. You should be fine, if you do have an issue you got BBF to turn too