Newer versions of Minecraft have been through multiple changes where world/chunk formatting is concerned, and as such worlds that were generated with or saved with newer Minecraft versions do not work with older versions. Most commonly this will cause your server to crash on startup, but even if it does manage to load it will have long term negative effects on your server.
Whether you have just purchased your server and changed it to the server version/setup you wish to use, or you have had a server running for a while and fancy a change, a new world should always be generated for the new version/setup to limit any potential issues that would not be there otherwise.
When A New World Is Required To Avoid Issues
- When switching to an older Minecraft version. Backdating servers has never, and will never, be supported - even going back between minor releases (ie 1.16.5 to 1.16.1) is not supported and can cause issues if you are still using the same world (or plugins etc....).
- When loading any modded setup for the first time (regardless of whether the world was previously being used with a different modded setup or with Vanilla). Using the same world with different sets of mods has a very high chance of issues happening, and should be avoided.
- When you load a snapshot for the first time. Even using a new snapshot for the same Minecraft version is recommended to have a new world generated for it (ie if you loaded a new world for 21w03a it is still recommended to then load a new world when you update for 21w05b).
- If you have been using snapshots and the actual release of that version of Minecraft drops. The world used with a snapshot should never be used with anything but that snapshot. As an example a world used on a 1.17 snapshot should not be used with 1.17 itself when it releases - a new world should be generated.
When A New World Is Not Required
- When updating to a newer version of Minecraft than the one you are using. This includes updating for a minor version bump (ie 1.16.4 to 1.16.5) or to the next major release (ie 1.16.5 to 1.17.*).
In this situation generating a new world may be beneficial (such as allowing for a world with the new chunk/world generation of the new release) but is not required as issues should be minimal at most when updating with the same world.
Note that once the world has been loaded with the newer version it should not be used on an older version, including the version you just updated from. If you think you may need to backdate your server later on then a world backup must be made before you update so you can roll back to it later on.
If you are running plugins however this adds more steps to update - you should not just update straight off without testing anything - All plugins you use must also be compatible with the new Minecraft version.
In this situation it is recommended to either run a copy of your server on a localhost or to make a full server backup before updating your live server and all plugins to their latest release and test for any issues (so you either have it running separate to your actual server, or so you have a backup you can roll back to if you have issues on the updated version that you need to hold off on updating for).
- When updating to a new version of the same modpack. You should check the modpacks changelog for the new version before updating however, as they may recommend a new world themselves to prevent issues the mod updates may cause (not likely, but it happens some times).
How To Generate A New World
To generate a new world you have two options:
1 - Enter a new world name in the World text field on your control panel, save changes at the bottom of the page and then load the server on your desired version/setup.
2 - Shut the server down, log into the server files and delete the current world folder you are using (download a copy first if you need it for future use). Load the server back up once the folder is fully deleted.
When loading any new setup on your server it is strongly recommended to stop your server, download any files you need for future use (such as world folders) and then delete everything before you install and load it on the new setup. This limits any possibility of conflicts caused by files from previous setups, as worlds are not the only thing that can cause issues across setups (particularly between different modded setups).