All you have to do is make a backup in recovery, do a full factory wipe them flash CM & gapps. However, keep in mind that if you want to go back to a TW based rom you will find that a lot of apps won't work properly. Even after restoring a nandroid I was having problems with apps not being able to access data (for example my weather widget couldn't d/l skins and my app market app couldn't d/l anything.)
To get things working normally on my TW backup I had to use Odin to go to stock MJ4. Then I had to do a factory reset within stock MJ4. Then I had to Odin TWRP recovery then finally restore my backup.
In short I really hope you like AOSP because if you don't it's a major PITA to return to TW
Sent from my SM-N900P using XDA Premium 4 mobile app
Free 15GB storage plus an additional 5GB for using my referral link. You can also get an additional 5GB for every person you get to join! Of course it has a Android app with features such as automatically syncing any photos/videos you have recently taken when you are on WiFi. Give it a shot!