Considering that the artist had a change of pseudonym, should we: replace all tags of trancy_mick with mick39, or just tag all his(?) works with mick39 as well? (Also, I'm not sure what is his actual pseudonym, mick39, or mosamosamick 39, or whatever)

Reference:
http://www.furaffinity.net/user/mick39/
http://www.furaffinity.net/user/trancymick/
http://mick.kemono.cc/mosamosa39.htm

His first nickname in characters "Western" was mick39 then she quickly changed trancy Mick and kept the nickname many years to change again recently has mick39

I do not know mosamosamick39 just mosamosa39 site ...

Because this artist is best known under the pseudonym trancy mick and the majority of his art is signed Trancy Mick, I suggest you leave this way.

In any case, aliases are to avoid ambiguity ;)