If I accepted an internship with Company A (but have not signed a contract), could I back out and work for Company B?

Background: I'm a college student interested in an internship in the media industry.

I've accepted an internship offer from Company A. I applied, they eventually got back to me and said yes, and I accepted in an email. I was told that contracts would be sent to me at some time in the future. As of now I have not received a contract but I have been told that I start in 10 days. Company A seems like a really fun place to work and it's in a field that I really enjoy.

Meanwhile, I applied to Company B and never heard back. It's the kind of place that never gets back to you. After accepting the offer with Company A, a friend of mine who works for Company B posted online that B was looking for someone to fill a late vacancy for this semester. For what it's worth, B is probably one of two or three "dream companies" of mine. My question is, if something were to materialize at Company B in the next couple of days and I were able to procure an internship offer at B, how unacceptable would it be for me to back out of my opportunity with Company A? How would I even go about it? I never signed a contract with A, but I gave them my word and I would likely screw up their plans. (Also, A and B are in the same industry, but cover different fields, so it's not like they'd compete for me or anything.)

As of now this situation is a hypothetical. For now I'm still an incoming intern at Company A and look forward to that. Still, my plan is to explore the availability at Company B and not make any moves until I get a firm offer from B (if I even do.) As a student, I could also intern for the other one next semester. Would it be best for me to try to convince one or the other to take me next semester instead? And if I do that, would it be better for me to do that with A or B? Am I going about this the right way? Thoughts much appreciated, especially as this is my first reddit post.