Question Regarding Political Affiliation for the Presidential Primary?

A question has come up among friends and family that have got us wondering. If a voter declares themselves a Republican during the primary does that mean that come November he or she is unable to change their mind and vote for an independent or a Democrat? I say that just because one votes one way in Feb. does not mean one is locked in to vote the same way come Nov. I also say that one does not have to officially change one's party affiliation to do so. Others disagree. Who is right?