Eating rotors or pads? Brakes otherwise working right?

Hard semi-metalic pads will eat rotors comparatively quickly. Soft metal rotors (aka cheap rotors) go even faster.

If the brakes are unbalanced hydraulically, you can end up working the fronts much harder than the rears. This will not be fixed by going to rear rotors. In fact generally this conversion requires an aftermarket adjuster to get the front to rear balance correct.

All by themselves, installing rear discs will do nothing to improve the fronts rotor consumption.

'97 T-100 SR5
'86 Toyota's, the variety pack (all gone)