Your AC is your class. You can confirm this by either hovering your mouse over any player's character, or doing a /who. You will never see a class of sith warrior-marauder, or sith warrior-juggernaut. You will see classes like marauder, juggernaut, assassin, etc. This tells me that AC's are NOT just "different specs" of the same class, but separate classes unto themselves. Therefore you should not now or ever be able to change from one AC to another, in my opinion.

Sounds to me like certain people waned an easy or quick route to 50 and so chose to level as DPS, but want to tank/heal for endgame. Gee, if only AC's had multiple possible roles so people could level DPS and respec talents at 50 to tank or heal. Oh wait, most AC's already have this option. I see no reason to allow a change of AC based on either of these arguments, since a viable DPS tree is available for every AC, making leveling easy while still retaining the option to change talents at level cap for utility. This is especially true since every AC gets at least one companion who can fill each role, DPS, tank or heals. Not all BASE classes get companions to fill a certain role at the same time, true. However, AC has no bearing on when you get that healing companion, that tanking companion or that DPS companion, since the companions and their arrival times are dictated by BASE class and story line. Thus there is no justification for allowing people to change their AC based on companion arrival time. If you choose a particular AC for quicker or easier leveling, that's YOUR choice. You made the choice, you should have to live with the consequences.