Changing server version, updating safely, snapshots, downgrades and version compatibility.
Changing version is safe as long as you know what moves with you. This guide covers what is kept, and how to actually change the version.
Downgrades are the risky direction, because a world saved by a newer version can carry data an older one does not understand. Read this first if you are going backwards, and check whether your plugins survive the jump before you commit.
Every plan includes a one-click version installer covering Vanilla, Paper, Forge and Fabric — see Minecraft server hosting.