Upgrading to macOS High Sierra is akin to walking on the bridge of peril. Too perilous!
I don’t recommend macOS 10.13.x for production, but it is necessary to test and for this reason back in September I did upgrade my test Mac. Of course, when the installer detects server it will give you a warning about it not being compatible and you’ll have to download a compatible version from the App Store. Be warned!
Which is no big deal as long you are warned and have backups and maybe you can download the compatible version from the App Store. Trying to launch the old version will get you a warning to go to the App Store and be quick about it.
Some people are reporting that the macOS installer is erasing their Server.app and refusing to upgrade their Server with the macOS 10.13 compatible version (v.5.4).
In that case, restore from Time Machine or other backups and start again?