From what i have seen it is nearly impossible to do.
Because the GID = the PID of the leader, so you have to change teh GID of the whole guild and all it's members, and alliances to the GID/PID of the new leader.
Kind of a pain to do, it is possible just a lot of work to be done.
Thats what i did and now :
My ally sees that (the new person) is Leader
On Cw proclamation all see (the new person)
But IN guild itself...the old person is still leader and the new "just" sub..and the sub (who should be leader^^) dont has Leader Rights so it aint just an "Wrong-Looking^^"
You also have to restart the server, any change made to the guild table in the database requires a restart, but good luck, your server won't start back up if you start messing with the guild table.