what is the difference in the HO that makes the extra 50 hp? is it the way the computer is programmed? if so could, the mag be reprogrammed to have HO #s.

No...Cam and ECU programming are different.

I thing the HO has forged internals (ie. Crank, rods, better pistons) where the 375 hp version has cast parts. In other words, major power increases would probably reduce engine life.

These days the engines have the same internals.
Cam and ECU are the only differences