I did it last year with my watch 3 as trade-in. They only want the watch. The charger can not be refused due to safety laws. Nobody wants a used strap.
Found a contact number for the company who mamage the trade in offer under samsung name. Called and was told only the watch unit is required and not to return the starp or charger. I had found a T&C doc where it said charger must go back too but they advised that the document was old and nonlinger valid. Hope this helps anyone else. Now it's time to orderbthe watch......