Wish to know the difficulty porting a device from one CM build to another(12.1-13)

Hey I have a device that hasn't received Android M yet and have built ROM's and APK's from source before just haven't done a port, what is the difficulty in your opinion of doing this? for clarification its a SM-t520 built around a exnos little big processor that is currently being maintained on on CM 12.1 and i wish to bring it to 13. :D

also if a chose the wrong area to post please point me to where i should be, i'm a lurker for the most part so I haven't really posted in the forum all that much.

Download from this link