I had purchased the server app many moons ago when it first came out. It shows as a new purchase for me now, so I'd assume you would need to repurchase it for every major version of macOS.
Didn't work that way for me. I bought it when it first became available on the App Store and now it requires a repurchase.Once you have bought server, the upgrades are free.
Unless Apple decides to charge for the upgrade. Post #4 right above yours details how it's worked in the past. We don't know if the High Sierra version of Server will be free or not.Once you have bought server, the upgrades are free.
Didn't work that way for me. I bought it when it first became available on the App Store and now it requires a repurchase.