The Guildmaster or Guildmistress can change the name of a guild, before the public, in all instances where it is mentioned by events, news and activity in the realm.
Syntax: GUILD SIGN <name of guild>.
Simply declare the new name of the guild by changing its sign in the guildhouse and thereafter the guild will be known by the new word whenever mentioned, and including references such as HELP GUILDS and guild bulletin board posts and guild-member referenced activity. Indeed a new sign is essentially a new identity and it has been used in the past to establish fresh reputation and outlook out of moribund past. The two guilds pioneering the sign change were Thieves - becoming Nightblades - and Craftmasters - becoming Sentinels. The Thieves have since returned to their original name.