In essence, no.

One of the citadels will be abandoned - citadels cannot be joined.

In practice, yes:

Step 1: Leave prior order.

Step 2. Join new order.

Done.

Quickest:

1. Have new order send invites to incoming members.

2. Members leave current order then immediately accept invitation.

Done.

Caveats:

- DO NOT ABANDON THE MASTER PLAYER ACCOUNT! At the least, assign Master to a caretaker account. Can be dormant, just can't be inaccessible. Else you make a boat anchor in your k that is just a pointless waste. Have someone make a second account, and post the name of it to the order info, or something. If something needs doing, so someone knows whom to pm.

- An Order can get along well enough without a Master, just don't lose all the Counsellors! The only things things that only a Master can do are demote a Counsellor, change the Order Code [no biggie, any Counsellor can change the Order Info], and distribute any RSS rewards from winning OvO. [The last is the only real irritant. Uninterested Counsellors will leave on their own, or go dormant - not productive, but at least not counter-productive.]

- It can be handy to keep the 2nd order around to help helping into Helping Enemies events. Each order can send a single troop to suicide against the other - gaining entry into Hunting Enemies. Including farms that never unshield, by having them start yielding a tile, and the other order sends one troop against it. [One troop, without hero, is not an attack - it's a probe or door rattling. Friendly neighbourhood watch checking that your doors and windows are properly locked.]

- the 2nd order can be handy for the more, or less active players. They can dominate, or not detract, OvO events. e.g. All farms / non-fighters could go to one or the other.

- the 2nd order can be farms for the other, since orders can't attack their own. This avoids the port tax and cart size limitations. (It also means other orders can drain the farms too, not just yourselves. You have to shield / watch carefully.)