Greetings,

It's possible to change Grand Companies, but you need to be the proper rank to do so. Once reaching the rank of Second (Storm/Serpent/Flame) Lieutenant, you'll be able to switch to a different Grand Company.

For those of you coming back to the game from 1.0 and have reached the time when you need to select your Grand Company, please be aware that if you choose a different Grand Company from that which you belonged to during 1.0, you will start from the bottom rank and have to work your way up.

After reaching Second (Storm/Serpent/Flame) Lieutenant and you make a decision to go back to your original Grand Company, the rank you attained with that Grand Company will be restored. (They'll welcome you back with open arms…most likely. )