They already have experience with this in their former mmo FF11, usually what happens is that certain features such as the jobs and levels can be kept without the expansion being registered in both systems and this is also assuming that you unlocked the new jobs and reached the new level cap; however, if you logoff in a expansion area you will not be able to log on again unless you register the expansion in the system that doesn't have it, or you would have to log on in the system that has the expansion and move to a 1.0 area.