If you transfer a 1.0 character into ARR, you can switch to only those classes you have already gained EXP in 1.0 in at the start (so if for instance you only had exp in say GLA, PGL and THM in 1.0, and you start ARR as say CNJ with your 1.0 character, you can only switch from CNJ to those other classes you already had levelled in 1.0.

Before you can switch to any other class you have to do the class quests for the class you started ARR as up to the level 10 one, and then go to each of the other classes' guilds in turn and sign up there to unlock them, like with a new character. This also is the only way you can undertake the class quests for those other classes you already had EXP in 1.0 in.

There is no way around this I'm afraid, as as far as I can tell this is 'working as intended', as it's been that way since ARR launched.