This is a technical forum, and license questions are not technical, but legal.
I believe that it is indeed possible to use a license for a newer version with an older version of SQL Server, but for an authoritative answer, you need to talk to a Microsoft representative. If you don't have one locally, you can call (1-800-426-9400), Monday through Friday, 6:00 AM to 6:00 PM PST to speak directly to Microsoft licensing specialist.