Yes, you get wristbands when you enter. If you are already in the park, they exchange tickets for wristbands in front of the castle. Last year, Tomorrowland was open between 6 & 7, they checked for wristbands at the entrances to the rides.
If you go through the turnstiles, they give you the ticket back since it was validated when it went through. I'm not sure what happens if you exchange it in front of the castle, they would have to have some way to validate it otherwise someone could have handed their used ticket off to you. I've always gone through the turnstiles after 4:00.