Should I backup my current NANDmin.bin just in caseIf you were on 11.2 when you made the backup, yes.
--------------------- MERGED ---------------------------
So back up my 11.3 and then put my 11.2 on my SD cardYou will be on whatever your system was on when you made the nandmin.bin backup. However in your case you might want to use the sysupdater cia to downgrade to 11.2 after making a backup. Then you shouldn't lose anything.
EDIT - Be sure to make a backup before downgrading, there is always a risk of a brick when downgrading and if you don't have a current backup then that could be a problem. Especially if it turns out the Luma anti-dg fix isn't enough on 11.3.