Bwah...........there have been changes, they just make more sense than mouthf**k.
I'm well aware of O'bannon's original backstory, but since it wasn't explained in Alien, I have no trouble accepting the queen in Cameron's sequel. The dogalien was an extension of the original movies ideas.
I'm less impressed with Ripley 8 and her other human/alien hybrid the newborn, but there was change there as well.
Of course this isn't even talking about the changes outside of the alien that affected the story and the main character, the main character also going through a logical progression in the series.
How much change you notice is how attuned you are to things OTHER than the popcorn factor/money shots.