The red R2... I like it. It fixes a continuity error and, IMO, was so well done that as I was watching I actually had to stop and ask myself 'was that droid always there?' (and this from someone who's seen ANH well over 100 times).
As for the added music to the briefing room... love it. In fact, I find it's the best addition out of all the added music (very subtle and, like ImpFighter said, makes the scene much more ominous).