Question about SodaStream Duo

Hello, I bought a SodaStream Duo system in February after our old appliance broke down. We are now sitting on four "old" screw-in cylinders, three of which are still fully sealed and unused. We wanted to exchange these cylinders for the new Quick-Connect cylinders, but the dealer won't take the unused cylinders back. Where can I exchange unopened, sealed Soda-Stream cylinders with a screw-in system for the new Quick-Connect cylinders?