Actually from my experiance if i swap classes at all when i have rested exp i lose all of it. Eg. I had about 1/3 of a bar rested on my archer at 17, went and got profs from limsa guilds, hadnt done any kills or turn ins or watnot, went back archer and had no rested. Also i didnt have any rested when i was on those lvl 1 classes either.