If i buy a coldfusion 2016 license, is it possible to install an earlier coldfusion version with the same license key
The non-authorative answer is yes, to a point. We currently do this with some CF9 servers we have. But then again I would strongly consider upgrading to latest unless there are extreme factor involved with upgrading because you are paying for a license that is most likely more that the version you are trying to keep alive. Also security, feature set, and speed are all greatly improved.
I'm not sure what version you are keeping alive but we had several compatibility (or lack thereof) when we moved from CF5 to CF9, but CF9 to CF11 was almost issue free (one or two small gotchas) and my CF2016 experience has been the same (only one minor gotcha so far). It's worth upgrading if possible.
PS. One caveat though, I just re-read your question and you specifically mention "same license key". Adobe can apply your license to a different version but it does require a new key from Adobe. That's where the email address above comes in.
Also, I believe the license downgrade can only be done by Adobe if you purchase your license through a retail or VAR - NOT directly from Adobe on the Adobe store.
Thanks Guys, this has been great great help.
Some of our applications are still using (mx7, CF8), i need to upgrade those to 2016 but if its not possible, thats where the downgrade to an earlier coldfusion will come in
You should also expect that Adobe will only provide downgrade keys for **currently supported** versions of ColdFusion (CF10 & CF11). You most likely won't be able to get CF7, CF8 or CF9 keys.