Two Symantec Endpoint server versions on same network

We do IT for a customer who has a endpoint protection server 12.0.1001.95 and the hard drive is to full to do any kind of upgrade etc.

So we have determined since he has kept up on his mintenance and I can and have downloaded the latest greatest we are going to install it on a new server.

My question being can I do that and just push the newer protection to clients and leave the old one in place till I get everyone up and running on the new one?

Or will the old server try to do something the clients after I have upgraded them?

Do I need to do a all at once and install the new server then shut off the old and push out the new protection?

Thanks in advance.

Mike