I don't know if I'm coming at this from a different perspective as a snorer, but I think B has very little legroom here.
1. I think even if there were another standard room available, B would be a little out of line for even asking A to cover the entire cost of the first room if they had previously agreed to split the cost of a twin room, let alone expecting it. Asking A to cover the entire cost of the original room plus half the cost of the new room would be so totally out of line that I assume the OP means the former (i.e. that they split the cost of both rooms). I still think this would be out of line as an expectation.
2. If B did ask A to split the cost of a suite and A agreed, I don't know by what grounds A would not be entitled to take the nicer room.
Snoring is not something people can control, it's not always something that people know they do, and it's sometimes exacerbated by sleeping in a different bed. I think if you agree to split the cost of a room with someone who you've not shared with before, you should expect that snoring is a possibility. And, as others have said, earplugs, headphones, are a better solution than changing room.