Effects of changing a Free Company's Grand Company

I want to swap my FC from the Maelstrom to the Flames because it's more convenient for me. Will my FC lose rank or any privileges? I'm sure we will lose our GC standing but I'm not even sure what that does. Has anyone gone through this process recently?

You will have to start from 0 reputation with the new GC and build it back up to unlock the rewards again. IIRC the old GC reputation will also go down as you do this.