Seems like if it is a boat with some legacy, selling them with the boat would be cool so they stay connected. Write your name or sign them to mark your place in history.
If they aren't going to be used by the new owner of the old boat, you should keep them and fly them on the new boat. But pick out the best of the best ones that you have memories of winning, and mark on the flags that they are from the old boat. And then as they get tired or as you win new ones you can replace them with new regatta flags. Say every time you get a new flag that you won with the new boat, you retire an