I know there's a discord channel dedicated for such inquires as yours above. I'm not sure if the splinterlands team allows for players to link to someone after signing up (I think they do but I'm not positive).

However I don't think they would allow for signups to switch affiliates as that would be a big No,No in the affiliate world.

One can still link to an affiliate regardless of if they already have a HIVE account or not. Clicking an affiliate link is all that is required. The first affiliate link you click is the affiliate that gets credit for the sign up.