Yes it IS common, but it is not common to the extent that these trailers were. People went from "videogame" comments to "Wow!" on the same shot. They improved DRAMATICALLY which is rarely EVER seen.
And no, by releasing a clip in this case they are saying "Here, take some free advertising! We have relatively none at this point because Universal and Marvel refuse to promote our film! See how action packed it is?"
And I do not think it looks very different, I think the skin on Hulk looks different. Which was improved from trailers, to TV spots, and since they most likely will be working on it up until the last minute all you can do in your case is hope they fix it as well.
Notice that many of the best shots are close-ups, meaning they are completing what they find most important first. (I.E. When Hulks through his shields at Blonsky.
Now am I saying you are wrong? No, because I very well thought the look of the film would improve from the trailers, but never as dramatically as it did. Who knows what is going on over there, we could walk into this movie and these scenes may look the exact same, or not. Only time will tell.