You can run a mixed environment, however, if you can't get them all upgraded quickly.
simply leave the stragglers running under a legacy SAV parent server, even a desktop computer acting as a parent server running SAV server on it.
Forward the logs from SAV to SEP and you can kill the SAV reporting server.
Then simply upgrade the clients as you can and they'll move to the new SEP system.
It's not an all or nothing deal.............