In theory, yes; I would have had to pay a price difference for a different fare basis. In practice I've never actually had to pay any price difference. No idea if I've been lucky, but I've done this repeatedly and always was able to change my return date free of charge. Just be advised that you can only do this once. I rebooked and asked to be put on a waitlist for another date during the same call once and was informed that the waitlist would have been a second change.