if it is a valid license , you can activate that on the vsphere editions , try to attach and assign the license
and the question for downgrading the vsphere.. it involves planning and the type of the of environment on which the vsphere is involved ,
1. take a back up of your vcenter data base
2. use the recovery option form DCUI shift+R
3, make sure the bootbank has the desired verison of Hypervisor(if you have choosen the upgarde option ,you will see the previous verison of esxi)
4. you can downgrade the vm hardware version using vmware converter
this should be helpful for you
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1033604
let me know if you need any help