Artist tag change

Artist -

SFM Inflation Artist
I'm having trouble with me getting a new artist tag? my old one is "artist:trentonstyle" and I want to remove the old ones (currently 2 with my old twitter name and fs account) and i want my tag changed to "artist:retrothegempony" but it says it doesn't exist? im just having some trouble and stuff and just want it to be changed
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