The specific details have been finalized. In general, levels are slightly more valuable than they used to be (a level 50 character after the change is a little more powerful than a level 50 character before the changes), so in general, most characters will lose a level or two.
Some classes will not lose any levels (Healers, for example) and some classes will actually gain some levels (Mages and especially Wizards) to reflect the huge challenge it is to level currently as those classes. Other classes will lose more than just a level or two -- particularly Barbarians and Paladins, also to balance and reflect how easy it is to currently level as one of those classes.