I am very particular about my F/C seating. So if I get my favorite seat that I would normally want and someone asks to change then I simply say " no thnks, I really prefer this seat." I don't do Bulkheads at all unless it is Buz class on CO and that is the only bulkhead I will sit in for the flight. I see no problem in denying to switch to a bulkhead seat cause they do have advantages and disadvantages. Futhermore, I hate to feel this way but they are in F/C and they should be happy with any seat since it is an upgrade otherwise they can be in coach together and I am sure two people would switch with them then. I have been given bulkheads at the last minute and I took them without hesitation if I wanted to get home. You shouldn't feel bad for saying no.