Okay, first off I'd like to inform you that I got my Note II just a few days ago so I'm not entirely sure yet how this exact phone handles certain things (e.g. KNOX built in to the bootloader) but I'll tell you what I think.
I can't really imagine different versions of the same ROM (i.e. Touchwiz-skinned Android 4.3) having a different bootloader (please correct me if I'm wrong!) so I assume that wouldn't be a problem. Or you should mean that you're on the 4.1.2 bootloader, in which case I'm not even remotely sure since I don't know the exact differences between the two (besides KNOX, that is).
Whether it's downgradable or not also depends on the bootloader you have, I guess. I've read that if you updated to stock 4.3 (through Kies or OTA), your bootloader has updated as well (to a KNOX-enabled one) and you aren't able to downgrade whereas if you are on, say, a rooted 4.1.2 version of even something like CM11, you might still have the old bootloader (in which case I'm not sure whether this ROM will fully work - aren't there Wi-Fi problems related to the old bootloader?).
Just keep in mind that I'm trying to answer your question with info I've found on my own (which may or may not be entirely correct). I just got this phone and although I'm fairly familiar with the dos and don'ts of flashing ROMs for the GT-N7000 (original Note) and the HTC Desire S, this is a new type of thing for me (especially with this whole KNOX thing).
For new things in this ROM: I have no clue. I have experienced that most versions MK3, NL8 or whatever are just different versions region-wise but they don't necessarily differ in features. Perhaps the more recent versions have some extra optimizations, but I'm really not sure.
Merry Christmas to you, too ??
Sent from my GALAXY Note II using Tapatalk
