In one way, I believe you are right.
Special Effects.
I'm sure Lucas didn't want to have to use the vasoline trick with the landspeeder, but he had to.
He wanted to have the dogfights look more intricate, but the technology wasn't there for him to do it.
But, some changes are not necessary.
The ROTJ ending.
The scream.
The shot.
The lead-in to Mos Eisley. (Dump the extra lead-in stuff, like the Ronto, but keep the changes to already existing scenes.)