In the Settings tab under 'Software & Version' you can switch between 9 softwares: Vanilla, Paper, Purpur, Spigot, CraftBukkit, Fabric, Quilt, Forge, NeoForge.
When a switch starts, your current profile (world + plugins + configuration) is backed up automatically; the new software installs clean.
Between plugin-based (Paper family) and mod-based (Fabric/Forge family) softwares, content isn't compatible, so a clean install is required — which is why the profile backup matters.
To return to the old software, switch back in the same section; the backed-up profile is restored and your world continues where it left off.