Based upon what other peoples experience is here on the forum then it will upgrade to a non-Server version and then you have to buy the Server App add on afterwards in the App Store to restore the Server functionality.
Yes you do have to buy the new Server app. Be sure to make a full (clone) backup first and verify that it boots and also make notes of all of your settings. Depending on what you are moving from it can screw up.
Going from Mountain Lion Server to Mavericks Server took about an hour for the server upgrade but was successful. When I previously went from Snow Leopard Server to Mountain Lion Server the system IP address, the DNS configuration, and the file shares were major messed up (and the automatic upgrade to Server ran for over 3 hours!).
A new Server.app release happened just today so YMMV.