well they certainly could do AC respecs, they were toying with the idea at one point. It would be a pain to regear, especially for a sage going shadow (single to double saber), but more so a guardian > Sentinel or vise verse there is a whole armor class difference there. Troopers wouldnt have so much problems (maybe getting a rifle/cannon) or agents, so class fairness wise it would be completely unbalanced. Not to mention the aditional cost of training the other ACs abilities
id be personally against it, since something about changing your core class bothers me, but there isnt a logical fact based argument against it if we allow dual specs