I swapped out the Subaru OEM pads about 7,000 miles ago for a set of EBC Greens. Now the discs are warped so I've got a pair of Black Diamond 12Gs on order. When I replace the discs, should I replace the pads as well (they look only about 50% worn, if that)?

No real need to, as lonmg as they aren't overly grooved or wearing at an angle. Some people reckon that new discs bed in better with older pads.

Just take some sandpaper over the surface of the pads to take off any small grooves, or glazed material and they should be fine.

Hi Carl,
As John says, there isnt a need to change but for the cost of the pads you may as well if your getting them done by a garage, more peice of mind and less to check later on!

Tony