Modifying an uid, can it be done?

[Ismael Valladolid Torres]

I edit my own public key:

$ gpg --edit-key ismael
gpg (GnuPG) 1.4.1; Copyright (C) 2005 Free Software Foundation, Inc.
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions. See the file COPYING for details.

Secret key is available.

pub  1024D/DE721AF4  created: 2005-06-20  expires: never       usage: CS
                     trust: ultimate      validity: ultimate
sub  2048g/689908B7  created: 2005-06-20  expires: never       usage: E
[ultimate] (1). Ismael Valladolid <i.valladolid at oberthurcs.com>

I'd like to edit my uid, adding my second surname and setting the
company I work for as a comment. However I don't know how to modify an
existing uid. Can it be done? If so, how? If not posible, why not?

Of course I can add new uids using the adduid command:

[ultimate] (1)  Ismael Valladolid <i.valladolid at oberthurcs.com>
[ unknown] (2)  Ismael Valladolid Torres (Oberthur Card Systems)
<i.valladolid at oberthurcs.com>
[ unknown] (3). Ismael Valladolid Torres <ismaeval at free.fr>

Looks like it would be enough deleting uid 1 and setting uid 2 and
primary, then it would "look like" I had edited the first uid. However
in that case I lose the "ultimate" tag to the left of the primary key.
What does this tag mean here? Which way to have this tag for the new
uids created?

Cordially, Ismael
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