I'd change the atrocious dialogue, awful acting, one-dimensional characters, lack of character development, numerous plot holes, scientific impossibilities, lame wisecracks and get rid of all the lame-looking model/puppet aliens (that doesn't mean all of them, just the lame-looking ones).
Generally I'd make the whole thing darker - darkness should permeate the films and it doesn't (you just get glimpses of it). The Riddick films are much darker plot-wise than any of the OT films, and the OT could do with a healthy dose of that darkness. After all, the rebels are meant to be living in this oppressive galaxy ruled by an evil Empire, but I don't get any sense of oppressiveness at all watching the OT.
Fanboy/fangirls will probably call for me to be burnt at the stake as a witch and denounce me for blasphemy, but deep down they know I'm right -- search your feelings.