Purchasing an apple watch as a gift

My now-husband currently has an apple watch but I want to buy him the latest one for his upcoming birthday as a surprise. Can I just buy one and then have him turn his old one in later for a credit or something? I know you can turn an old one in when you go to buy a new one but I didn't want to tell him ahead of time. If I can do this, is there a time frame that I must do it in, like 2 weeks from purchase or something?