No one is assuming anything, LL said himself it is unfinished. The hulk won't look drastically different design wise no, however skin texture and blending into the background moreso are things that can be drastically changed in 3 months. Look at the tank scene in the Iron Man superbowl trailer, looked very fake, videogame like and completely unrealistic. Only one month or so after the release of that trailer we got the full trailer featuring that scene, however looking completely different, very realistic, great. TIH had about 3 months or more depending on when the shots were given to the editing team to improve on the cgi, plenty of time to make big green look better.
As for why release it with unfinished shots, LL was under pressure to show something, many argue he was already really late in the game, so I don't blame him releasing unfinished shots at this point.