Blizzard does not assist with transitions in guild ownership. Instead, Guild Master Dethrone allows guild leadership to pass to active members in specific situations.
An active guild member can use the Dethrone function to claim the Guild Master rank. When the current guild master has been offline for at least 90 consecutive days, the Dethrone option appears with a notice to eligible members in the Guild News & Events feed.
Note: The 90-day timer only considers the login activity of the guild master character, not the account that the character is on. Logging in to other characters on the same account does not reset the timer.
Eligible members must be guild rank 1, 2, or 3 and must have logged in within the last week. To see your numeric guild rank, find your guild on the World of Warcraft community site and view the Roster. Higher rank characters get higher priority for notifications.
Note: It may be possible for ineligible characters to see the dethrone notification. If these characters try to claim guild leadership, it will fail and they will receive an error.