In Paxos, can an Acceptor accept a different value after it has already accepted one?

Question

In Multi-Paxos algorithm, consider this message flow from the viewpoint of an acceptor:

receive: Prepare(N)

reply: Promise(N, null)

receive: Accept!(

Answer 1

(Answering my own question.)

My current understanding is that I should not to accept the value in N+1 (i.e. not answer at all or send a NACK), thus forcing the leader to possibly start another round with Prepare (if the majority hasn't reached consensus yet). After I receive Prepare(N+2), I shall reply with Promise(N+2, V1) and continue as usual.