When I used to do bumper stickers, I would buy magnetic vent cover sheets and stick the bumper sticker to the magnet sheet. Then, I could stick the magnet to the car and swap out bumper stickers at will. The magnet sheets come with a white layer on front. If you peeled off that layer carefully, there would probably be enough adhesive residue to hold the bumper sticker in place. Or, you could use a spray adhesive. ...This is all probably more expensive than just buying a new sticker, though.