From the link provided by Pup Patrol:
If a username already exists and has ever been validated, or the username has been used in any way, then the username is not available.
Therefore the idea hinted at by Chrysalides wouldn't work; since "Waldlaterne" has been used, then it's not available.
Spinning the idea further I'm worried if I go from Waldlaterne to ABC, then I won't be able to change back because, see above, Waldlaterne has been used (yes, by me, but does the Software check that??)