If you have a licence for C6 and then you the subscribed to the created cloud all you need do is cancel your CC subscription. Your still a licensed CS6 user and you can re-install CS6 if you un-installed it. If you did not it should still be activated and operational on your machine.
Some of you new assets may not be fully useable with CS6 though.
If you had CS6 as a subscripton it should be still active unless you deleted it when you installed CC.