I think the very first time you ever change GC is free, but 50k gil for every change after that. You will retain your GC rank,PVP,seals etc but be unable to access any of them until you switch back.
So for the GC you just joined, yes, you start out at zero seals, and have to rank up from the very beginning again.
Also, any items you have that are glamoured with a GC specific item (ie: Allied Seals GC gear), will show their default, non-glamoured skin. And chocobo will be unable to equip the old GC bardings.