You don't run both. You stop the main location, and change the announcement tag, your PF's, your shoutruns and your flyers to the new location. Making a new character is 1 day of leveling to reach level 50 to buy personal housing because the new worlds all have massive xp buffs. FC housing doesn't even require level 50 so it's barely a few hours just to reach grand company level to create an FC. As for progress on the FC, none of that is remotely important to baseline functionality. Your submarines/airships still exist and can be used, but don't function as a part of venue openings, same goes for crafting. The only part that is usable during an opening is the private chambers, and if your staff was paid properly, they should have their own apartment/housing instead of being reliant on the one's you had. The venue provided rooms are meant to be there for QoL, but not out of necessity. Most staff will eventually decorate their own housing for their specific type of rp anyway so a chunk of them that don't live on congested worlds are going to be fine. For the one's that are on congested places, most venues don't have more than 3-5 sessions happening at one time. Even if you were high end and had 10 sessions overlapped, cancel 1-2 openings, spend that day leveling with everyone in party that would've been at the opening anyway, and hit level 50 and be done. 5-10 characters to level 50 when you should have a staff line up of 20-30 people by the time you hit that level of concurrent demand, is not hard. Any of the staff being bought consistently opening to opening should have plenty of gil to make their own apartment/house on a non-congested world and not need to use the baseline 5-10 rooms a venue has. Not to mention they'd want their own place anyway to be able to lock the building since most have out of opening rp sessions as well that'd be affected if they were in a congested location. Most venues don't even start with more than 3-5 rooms if they have any at all. It's only a gil check. You wouldn't be out of commission for months out of necessity. You'd be out of commission for months due to laziness.
You just answered your own question, who fixes something they don't care about at the moment.