How to change your Minecraft server version or software

Changing the version or the software of a server is two different jobs that look alike. Version means 1.20.4 to 1.21.4. Software means Vanilla to Paper, or Paper to something else. Both are safe if you prepare for them.

Before you change anything

Back up the world. Always, see how to back up a world. A world opened by a newer version cannot go back to an older one.
Check your plugins. Each plugin is built for a range of versions. The plugin compatibility checker tells you which have a build for the version you want.
Check Java. New versions can need a newer Java, the Java version checker shows which. Hosting services pick it for you.
Tell your players, so they update their game if they need to.

Going up a version

Newer is mostly safe: Minecraft upgrades the world as it loads it. Vanilla and Paper do it the same way. Expect the first start to take longer while chunks are converted, and expect some plugins to be missing until their authors catch up. The detailed steps are in how to update without losing your world.

Going down a version

Not supported. The world data of a newer version is not understood by an older one, and the server refuses to load it or damages it. To go down, restore a backup made before the upgrade. On Saryno, switching version makes a safety backup first when your plan has backups, and the Settings tab can roll back to it.

Changing the software

Vanilla to Paper: the world carries over. Paper keeps nether and end in the same folder layout on current versions and adds plugin support and speed. Players notice no difference except better performance, and some farms behave slightly differently.
Paper to Purpur or similar forks: worlds carry over, configs mostly too. Read the fork's migration notes.
To Fabric or Forge: mods are not plugins and the two do not mix. A modded world cannot go back to a plugin server without losing mod content. See Paper, Vanilla, Purpur or Fabric before deciding.

On Saryno

Open Settings, then Version, pick the software and version and confirm. A list shows how many of your add-ons are available for it first. Add-ons with no build for the new version are left out and listed, you can add them back when they exist.

When it goes wrong

If the server will not start, read the log with the crash log analyzer: most failures are a wrong Java version or a plugin built for another version. Remove the plugin or restore the backup and try again.