I am sure it has to work since newer versions always support older versions. But I guess the only way to know that would be by testing it.
That doesn't necessarily have to be the case when it comes down to Linux distributions. Packages may not yet support the OS.
Well for pw server it worked like charm, it did change some things but I still managed to reinstall pw on newer ubuntu version. But for this one I can't even test because it seems that all files that were in .rar files uploaded somewhere have been outdated.
If I am wrong please pm me links, but as far as I can see I can't get my hands on clean server files atm.
Like I said, most packages get updated at the same moment a new distribution is released. Considering neither of us knows what packages are being used for Forsaken World, all we can do is let OP give it a try.
Besides, you don't need to re-install Ubuntu in order to upgrade, update manager would do.