As was mentioned, you should be able to push install the 9.1 agents from the media server. I have never had any problems pushing out the 9.1 agent over the 8.6 agent. So you should not need to uninstall the 8.6 agent.
If you do have to roll back to the 8.6 media server, you will need to uninstall the 9.1 agent on each server, and then reinstall the 8.6 agent. I believe that when you go backwards like this, you will need to reboot the remote machine after uninstalling the 9.1 agent, and before you reinstall the 8.6 agent.