Agents are specific to each version, with some backward compatibility. This is as far as the software itself is concerned.
As far as the license keys are concerned, I think they are version specific.
As your BE server is also your Exchange server, when you upgrade BE it will also upgrade the Agent, so you will need to have a V10 license key for the Exchange agent.
I'm not sure what happens if you try and upgrade without a license key for the agent.