How does the transitioning to another Grand Company work?

[Yamimarik]

You don't lose your rank at all, when ever you wish to transfer over back to your old GC, your old rank will still be there for that GC. You start out as a new GC rank in a new GC, you'll have to pay a 1 time fee of 50,000 gil. And you must wait 15 days before you can transfer to another GC again.

[Coatl]

Nvm googled it and you keep allied seals