I have a similar Skype issue. I have ancient Skype accounts with usernames, and my email address linked, but now use a Microsoft account for Skype with that same email address. Nobody can find and add my current account - whenever they search for my email address, all of my old usernames appear. I always have to add other users instead.

Very frustrating - fortunately I don't use Skype regularly but it's always an exercise when somebody asks for my username.