I read somewhere that you are supposed to be able to rename your guild once per month but I cannot seem to find the article or post I saw it in. Possibly it was for a different game as well.
it is for this game in the f2p notes some where. i dont think it has been implemented yet. with it it also said character names and legacy too i believe.
if your guild has the same name of another after the mergers, ie has a server name behind it, then you get a free guild rename.
personally when any of these are done i wish a server wide in game mail will notify everyone so we can update our friend and ignore lists.