I don't know about your basic question, but need to ask you something else. Have you checked the specs on your server and compared them to what is required for server 11? The software and hardware requirements for server 11 will be quite different from what would work for server 5.5.
In our shop we had to buy new servers as the older ones just didn't have what it took to run server 10.
Our servers are fine for running Server 11. The only problem is that we have old databases that need to be hosted on server 5.5 and new databases that need server 11. I have 2 servers so there wouldn't in theory be a problem installing one version on one server and one on our other server, my only concern is that Filemaker say not to install Server 11 on a server which is also running Exchange which means server 11 has to go on our other data server and hopefully I can put Server 5.5 on our Exchange server but I don't really want to take the chance of trying to install 5.5 on our Exchange server in case it screws something up.
My only other option is to try and convert the old database to Filemaker 11 and see if it works, hopefully there are no compatibility issues (this all stems from my employers being tight with money and not getting the original converted and updated professionally!)