I think you might have to be patient and a leader will step in as in this post:
https://ubuntu-mate.community/t/is-it-possible-to-change-a-nickname-here/26836
There AFIK a lot of varying factors as forum is run on a subset of Discourse software based on needs and features determined by this forum's leaders.
Also it is run by volunteers who depending on software and personal obligations for free time will have to check to make sure new name is available and not sure what is involved moving your profile to new name whether automatically or manually. Don't think it is a one click operation. Think as in linked post a moderator will step in in same manner.
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