Hi George,
I believe that the client needs to buy or upgrade (if this is possible) the available licenses to the last version to be legal... If your question is about using asa 9 client to connect in a v. 16 server, then the answer is, it should work, but only functionality from asa 9 will be available in the client side. Actually, in the past we had an issue, where a 3rd party application was not able to communicate with an sql anywhere server v.11 using v.11 client. But it was working fine to connect using obbc client from asa 8! We do have a cpu license which has no limit on the clients... So we used asa 8 to connect through the 3rd party app, and 11 client to connect through our application.
And the remaining question is if it's legal or not... and I'm not the appropriate person to give an answer...
Andreas.