Watch the video:
Timestamps:
0:00 - Explanation
0:31 - Vanilla server world
1:00 - Modded server world
1:14 - Migrating - Backups
2:18 - Migrating world & data from Vanilla to Spigot/Paper/Bukkit
3:50 - Note on Different versions
4:13 - Testing if world migrated
Want to migrate your world and items from a Vanilla Minecraft server to a modded one? Well, it’s a little confusing the first time around but incredibly easy to keep everything exactly as it always is, but still install mods and a lot more, using Spigot, Bukkit, Paper or any other modded server in Minecraft.
This simple guide shows you how to migrate.
Allow clients with different versions to join: Go to post or View on YouTube