While onboard, you are only able to assign the same TA that you are using for the current cruise, if you are using one. If you did not book the cruise you are on through a TA, or you want to switch to a different one, then you have to transfer the reservation after you get home. It did not use to be this way, but when I was onboard in November this was the case. I was surprised by the change so I asked a lot of questions and this is how it was explained to me.