Is my user-id locked?

Since when have we been unable to change our user profiles?

I gave a course on Friday and one of the users missed the Sept 2014 call to upgrade to the amalgamated site so had an underscore user name. However, when he tried to remove the underscore we discovered that our user names are now locked.

Since when was that? We always were able to change our names (provided that the user name did not already exist) previously …

I don’t remember being able modify my User name - I have ONLY been here since 2013.
But I still have three usable Identities - the underscored one from the ZA is from the period you mention. At the time, or not long after, you gave me the impression that it was a difficult problem to get it ‘reverted’…Locating that comment would be a good search-challenge.
I suppose it ought not to be locked but imagine the difficulty encountered by the code in the quest to change all the references, in all contexts, from one to the other (if it was necessary).
Interesting to note though, that my Signature, which I change frequently, is amended in ALL my comments and posts, way back to the first and earliest ones - probably now involves over 9000 occurrences (I can ‘hear’ you grunt at such a low score @Tony_Rebelo)

Trivial: iSpot remembers your user-ID - a number. It populates your name on the fly from that link. Prominent users who changed their names were Beetledude to Riaan Stals, Riana Bates to The Bate’s - there were others.
Changing your name is no more problematic than changing your signature or photograph.
And yes, changing your signature/photograph/user_name will change everything, because all that iSpot knows about you is that you are user/21890 - the rest it gets from the User-Lookup table on the fly when someone looks at your observation/comment/project/agreement. Similarly your reputation and society badges are the current ones: not the value-societies when you posted the ID/agreement or comment - iSpot looks them up on the fly. That is why if you “refresh” an ID by agreeing and then removing your agreement, the Likely-ID may change, depending on how much the different user’s reputations have changed since iSpot did the last evaluation (the last time it had an agreement or ID - but it only recalculates the reputation for the ID you made your agreement to, not the others, so you can squeeze a few extra points for that particular ID by doing this).

The problem with your underscore Name is that you cannot join users, so your comments/IDs/Observations will always stay under the different user, no matter what you change the name to.
And each different name has to have a different email.