It is still a little vague about whether they'll stay on the same world, if they don't subscribe during the merging process, but due to what they said, and they backlack against randomly placing unsubs, I would think they'd just merge into the world that server they were on pre sub is merged into. I think the merge open and closed status was just a percentage of the actual room available on a server. I mean pre ffxiv coming out they said the server's were made for 10,000 people. I would assume there servers can still technically hold that many, the functionality of the game just drops severely due to lag. But yet again this is all speculation from what they said, and not really hard evidence, cause only SE has it.