If the old bearings are ok they can be put in the new rotors. I think new
bearing/race sets are no more than $30 a side. An auto machine shop can
press in/out the races. They might also pack the grease and install the
seal. I have done this on the driveway myself using screwdrivers drifts and
hammer. One time the place I bought the parts insalled the races and packed
the bearings for no extra charge. Just assembly after that.
good luck- Bob