Sure, but they should be upfront with that when booking. Fwiw the plane seemed fairly full when picking seats, especially when taking into account the blocked out the middle seats.
I would always wait until everyone boarded, then move up to comfort plus or whatever it was called. On my last flight the attendant told me to move back since it wasn't fair for the suckers, no offense, that paid for it.
I offered to pay a nominal amount for the seat. The attendant pulled up the little register stick and said it would be $75. I said no thanks and moved back to my empty row about 3 rows back.
I guess some airlines do it differently.