Okay, so moving the old rears to the front will wear them down sooner. But putting the new tires on the front will also wear those down sooner. Either way you're shortening the lifespan of a pair of tires.

It's not like if you put new tires on the front and older tires on the rear you're going to magically turn your car into some kind of tail-happy unsafe road sled. It just decreases your margin of safety in emergency driving situations.