As long as you're not checking any bags, you can standby for an earlier flight at no charge. If you are checking bags, the bags may or may not make it onto the earlier flight. If you already checked your bags, then they will most likely NOT pull them from the later flight.
If you want a confirmed change. I think the change fee is $50.
I almost always check a bag with JetBlue, so I've never attempted to standby for an earlier flight since I'd be stuck at the destination waiting for my bag anyway.