tried a few of these out in Firesupport before i got my UK1, the front opens like that to access the battery. From memory, yours i believe is also rear wired like my Transform4 short version, or did you get a front wired one?. i think the HOG is only front wired anyway hence the opening compartment
The upper receiver of my UK1 is compatible with the HOG, but im not sure the longer version is, cant see why it wouldn't be, its just a longer barrel. Although there is very little difference except the keymod type and as mine is rear wired its not of any use to have the opening battery compartment.
You could buy the HOG upper receiver and swap it with yours, you'll end up with a shorter gun, but if you have a rear wired version, i wouldn't spend the money, unless you want a shorter AEG
if in doubt though just give Firesupport a call there may be an easier way and they can give better advice