I think that is a question you will need to ask a FMI sales rep about.
If you plan on upgrading to FMS14, 15, etc...I would consider the annual licensing. It is 1/3 the cost of buying Server, and connections. And the upgrade carries the license with it to the next version ( concurrent connections included ).
This soon after the release of 13, I would doubt that they are ready to talk about pricing for the next version(s).
With the annual licensing, it seems pretty obvious that whatever you are "subscribed" at will continue forward.
But for those who prefer to still "purchase" their licenses, rather than "rent" them… These are good questions.
I can see the logic that concurrent connections are kind of like "lite" versions, since you don't pay for FMP with a browser or Go. So, it would stand to reason that there would be an "upgrade" cost to these things as well. You could only guess that an upgrade to your concurrent license would be about 55% the price of buying new, given the same approx ratio for new/upgrade copies of FMP. So approx $495 for your 5-connection upgrade?
This is all speculation, of course. One thing that we have learned from the v13 upgrade is that you can't just go on historical trends anymore. It will be interesting for sure, but for now, we have enough to digest just with v13, without worrying about v14, 15, etc…
If you maintain your annual maintenance, you would be covered as well.
Also, FM Server 13 can technically support more than 50 WebDirect users, but that is the maximum that FMI is recommending. If you have a mix of Web and FMGo users, I could see more users working, as FMGo is less demanding. If your app. is light and well designed, and your server is uber powerful, you may very well get more WebDirect users to work. You can purchase an unlimited license for a few hundred dollars more I believe (right around 11k, if I recall).