I got my shipment of EBC green stuff pads for the front of my car but the back has regular semi-metallic pads on. They dont need to be changed and theres still a good amount of pad left. How much of a safety concern is it if I leave them alone and only replace the fronts with the EBC's?

 

If the fronts are worn out, Go ahead and change them. Don't be surprised if the rears wear less quickly; they have less load. If it bothers you, change them now. If it doesn't, and/or you don't have the tool, change them later. I'd leave them until they wore out, unless you are going to be on a track and need the improved braking you'll get with the better pads.