doubleKO said:

Is it a popular theory that Luke was attacked by the wampa early in Empire to help "explain" this difference in his appearance?

SilverWook said:

That theory is as old as Empire itself. Yet we see Luke in closeup before the Wampa attack.

An unpopular theory then? I know it isn't new and that there are close-ups of Luke before the attack, but that doesn't entirely disprove the theory. An audience seeing Empire for the first time have a limited window to notice any difference before that event takes place, and it is a plausible explanation for those noticing afterwards.

It would have been too heavy-handed to have had it happen without clearly showing Luke at least once. He is injured in his first scene, I don't think it is incredible that this may have been a decision that was made to partially bridge the slight gap in continuity caused by his change in appearance. Or not...