hugs I have posted about this in AIBU because unfortunately there has been some fallout, yes, but consensus seems to be that this is unavoidable unless you are very lucky! You need to do what's best for your children though, and if he is unhappy then move him, and don't feel guilty. Ds2 has amazed me with how quickly he has settled at the new school, and honestly he is like a different boy when he comes out at the end of the day, like the weight of the world has been lifted off his shoulders, and he wasn't being bullied - he was just stressed out by the experience of being in a noisy, large class.
Is your younger son happy at current school? If he is then he presumably won't mind staying where he is, and you can put him on waiting list and see what happens. If he is unhappy that's a different matter. Do you have any sway with admissions given that you are going to be working there?