I found and enabled a setting that lets you change your username in Discourse, regardless of how it’s set in WordPress. Apparently the single-sign-on from WordPress relies on an ID, so changing the username shouldn’t break the connection in any way.
Once you’re logged in here, click your avatar in the upper-right corner and then click the little gear icon in the drop-down menu. The resulting preferences screen has your username on it, and you can click the pencil icon to open it for editing.
@tidbits1 Give this a try (though, ironically, as soon as you change it, this mention will break. :-))