The writers made a good point in one of the on-line Q&A's on the offical web site. The question was in regard to Bumble Bee being a Camero instead of a Beetle. The writers responded with basiclly its not the suit that makes the man but the man himself (not a direct quote). So just because the look of a character is different doesn't mean that the character himself is different. The writers said that they had the original media in their heads when writing this movie. So with that said, I guess that its alright that they changed the look of the decepticons b/c if the character himself is there that I guess thats all that really matters.
http://www.transformersmovie.com/main.html