Did it TWICE in the states when they came out, trading my S2 and S3 for two copies of the S4 watch. Unpair the old watch and have it ready. Tell them you want to trade in on new watch BEFORE they charge you for new, and you should only have to leave the watch case. They should deduct trade in right in the sale.
You keep band and charger. I recommend doing the check on Apple's site to see what they will give you for it so you know ahead of time. Good luck.