How would I go about putting all of my stuff on my current forum (posts, threads, users, etc) on a 1.4 beta? I don't want to use the beta as my live forum, I just want to transfer (or copy) everything over to test some thingsm. I tried once, I didn't do it right though..
upgrade your 1.2.x forum to 1.4 by install/upgrade.php file in MyBB 1.4
Create a duplicate installation of your 1.2 forum (copy sql into a new DB and files into a new directory), make sure everything is running correctly under 1.2, then replace the files with 1.4 beta and run the upgrade script as arno says.
I did all that, but I'm currently running on 1.2.13, but the upgrade script only supports 1.2.12 and lower
Yes, just run it from 1.2.12