Guild Faction Change Service, How does it work in 10.1?

[Sendryn-sentinels]

This should give you the information you need. Also keep in mind that the guild leader must be the same faction as the guild.

Guild Services

Some things to remember with cross-faction guilds:

• Guild affiliation with the Alliance or Horde will depend on the faction of the Guild Leader, and guild achievements and vendors will still reflect the guild’s primary faction.
• Guild members of the opposite faction will benefit from unlocked shared perks but cannot contribute to specific achievements. As an example, Alliance members of a Horde guild could not contribute progress towards the “Alliance Slayer” guild achievement.
• Guild repairs and social conveniences, like shared chat, will be available to opposite-faction guild members.
• Players must be Battle.net friends or part of the same Battle.net community to invite or receive an invitation to an opposite-faction guild on the same realm.

Play with More Friends with Cross-Faction Guilds

Vrak followed up in another thread with this:

@Vrakthris

The Guild Faction Change service still works the same, so when initiated it will still create a placeholder guild.

As noted in the article Sendryn linked, you’ll lose progress on incomplete achievements. Your guild bank will go with.