Major impacting is basically when there are too many applicants and students in a particular major. Usually, this means that there are specific requirements for admission to the major, and it's incredibly difficult to get classes for major requirements. It also means the standards are usually higher, and that it's hard to jump from one major into that particular impacted major. As far as the Viterbi School goes, it seems none of the majors are impacted, so you're welcome to switch between them as long as you fulfill the requirements, I believe. I'm unsure of the other schools.
Thanks for that bit about the transcripts.