The ServerQuery documentation lists client_nickname as changable.

When using clientedit the following happens:
clientedit clid=3 CLIENT_NICKNAME=Test
error id=0 msg=ok

But the nickname is not changed.

There seems to be also an inconsistency between lower and upper case properties. "client_description" works, but "client_nickname" shows:
error id=1538 msg=invalid\sparameter