Aussie Server

[Seraphor]

How was the housing situation during the EU immigration? Free transfer for those on Tonberry who already own a house to different servers that the housing are already full? I think it would make much more sense to offer free transfer to the new fresh server with free spaces.

With EU, the 10 servers were split right down the middle and an extra 1 added to each DC for 6 each. So there were plenty of people moving from one to the other, it was very much a two-way street.
The new servers Spriggan and Twintania had their housing plots locked for a few weeks to start with, to give everyone enough of a chance to relocate before they started fighting over all of the new housing plots that suddenly became available.
Those new servers are still the least populated of all servers though with just 30k (6k active) characters each, the next lowest is Omega with 73k (9k) characters.

(If you mean when EU immigrated from NA, I've no idea, that happened in... 2015/16 I think? I don't know what housing was like then)

With OCE, it's mainly going to be people moving to and from Tonberry, whether Tonberry gets moved or doesn't. But some AUS players do play on different servers, it's not all just Tonberry and Kujata.
So if they moved Tonberry over to OCE, then JP players leaving Tonberry could find empty plots opening up on the other servers that AUS players are leaving, such as Kujata, etc.

But as Layla said, I think it's very unlikely they would open up an entire data center with entirely new servers, and force only the OCE players to all move to a new server. Not only would it seem a little unfair, but you are bound to get some players who don't want to move, they don't want to leave their friends who are staying behind. In order to get enough of the AUS playerbase over to the new data center, they need to give it some grass-roots, and I think Tonberry would be that server to help populate the OCE data center. And depending on the OCE population of Kujata, possible Kujata too.

It's also worth noting that the JP servers arent anywhere near as overpopulated as the EU/NA servers.
JP server avg pop is about 45k (8k), while EU avg is 80k (10k), and NA is 100k (12k). So JP players probably don't find the housing crisis to be so extreme. I know on many EU servers it's not that hard to find a small plot, and the newest servers still have loads of empty plots.