Why LazyDFU Does Not Break Old World Conversion

How the Deferred Loading Works

Minecraft normally pre-builds every rule for upgrading old worlds before the window even appears. LazyDFU stops that pre-building and instead constructs those rules only when a world genuinely requires conversion. The rules are not removed or altered, just postponed. When you open a world saved in an older format, the conversion system still runs exactly as vanilla would.

Testing With an Old World

If you want to be sure, back up a world from an older version and open it with LazyDFU installed. The game should detect the old format, build the needed conversion rules on demand, and load the world normally. You will not see a special message; the process is transparent. The only difference is that the rule-building work happens at load time rather than at startup.

When Conversion Might Feel Slower

Because the rules are built on demand, the first time you open an old world the load may take slightly longer than a modern world would. That is expected and is the trade-off for a faster overall startup. Once the world is converted and saved in the current format, subsequent loads are normal. Keep backups before converting any important world, as you should with or without this mod.