A ForcedLevelPatch is always executed against level.dat, (no matter what patchLevel is set for a world)

src/main/java/ru/bclib/BCLib.java

@@ -12,7 +12,6 @@ import ru.bclib.api.dataexchange.handler.autosync.HelloClient;
 import ru.bclib.api.dataexchange.handler.autosync.HelloServer;
 import ru.bclib.api.dataexchange.handler.autosync.RequestFiles;
 import ru.bclib.api.dataexchange.handler.autosync.SendFiles;
-import ru.bclib.api.datafixer.DataFixerAPI;
 import ru.bclib.config.Configs;
 import ru.bclib.recipes.CraftingRecipes;
 import ru.bclib.registry.BaseBlockEntities;
@@ -50,8 +49,7 @@ public class BCLib implements ModInitializer {
 				Chunker.DESCRIPTOR
 		));
 		
-		DataFixerAPI.registerPatch(() -> new BCLibPatch());
-
+		BCLibPatch.register();
 		Configs.save();
 	}
 	

src/main/java/ru/bclib/BCLibPatch.java

@@ -1,14 +1,23 @@
 package ru.bclib;
 
 import net.minecraft.nbt.CompoundTag;
+import ru.bclib.api.datafixer.DataFixerAPI;
 import ru.bclib.api.datafixer.Patch;
 import ru.bclib.api.datafixer.PatchFunction;
 
-public final class BCLibPatch extends Patch {
+
+
+public final class BCLibPatch {
+	public static void register(){
+		DataFixerAPI.registerPatch(BiomeSourcePatch::new);
+	}
+}
+
+final class BiomeSourcePatch extends Patch {
 	private static final String NETHER_BIOME_SOURCE = "bclib:nether_biome_source";
 	private static final String END_BIOME_SOURCE = "bclib:end_biome_source";
 	
-	protected BCLibPatch() {
+	protected BiomeSourcePatch() {
 		super(BCLib.MOD_ID, "0.4.0");
 	}
 	

src/main/java/ru/bclib/api/datafixer/ForcedLevelPatch.java

@@ -0,0 +1,47 @@
+package ru.bclib.api.datafixer;
+
+import net.minecraft.nbt.CompoundTag;
+import net.minecraft.nbt.ListTag;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+
+/**
+ * A Patch for level.dat that is always executed no matter what Patchlevel is set in a world.
+ */
+public abstract class ForcedLevelPatch extends Patch {
+	protected ForcedLevelPatch(@NotNull String modID, String version) {
+		super(modID, version, true);
+	}
+	
+	@Override
+	public final Map<String, String> getIDReplacements() {
+		return new HashMap<String, String>();
+	}
+	
+	@Override
+	public final PatchFunction<CompoundTag, Boolean> getWorldDataPatcher() { return null; }
+	
+	@Override
+	public final PatchBiFunction<ListTag, ListTag, Boolean> getBlockStatePatcher() { return null; }
+	
+	@Override
+	public final List<String> getWorldDataIDPaths() {
+		return null;
+	}
+	
+	@Override
+	public PatchFunction<CompoundTag, Boolean> getLevelDatPatcher() { return this::runLevelDatPatch; }
+	
+	/**
+	 * Called with the contents of level.dat in {@Code root}
+	 * @param root The contents of level.dat
+	 * @param profile The active migration profile
+	 * @return true, if the run did change the contents of root
+	 */
+	abstract protected Boolean runLevelDatPatch(CompoundTag root, MigrationProfile profile);
+}
+

src/main/java/ru/bclib/api/datafixer/MigrationProfile.java

@@ -57,7 +57,7 @@ public class MigrationProfile {
 					 List<String> paths = patch.getWorldDataIDPaths();
 					 if (paths!=null) worldDataIDPaths.put(modID, paths);
 
-					 if (applyAll || currentPatchLevel(modID) < patch.level) {
+					 if (applyAll || currentPatchLevel(modID) < patch.level || patch.alwaysApply) {
 						 replacements.putAll(patch.getIDReplacements());
 						 if (patch.getLevelDatPatcher()!=null)
 							 levelPatches.add(patch.getLevelDatPatcher());

src/main/java/ru/bclib/api/datafixer/Patch.java

@@ -31,6 +31,11 @@ public abstract class Patch {
 	@NotNull
 	public final String modID;
 	
+	/**
+	 * This Mod is tested for each level start
+	 */
+	public final boolean alwaysApply;
+	
 	static List<Patch> getALL() {
 		return ALL;
 	}
@@ -82,6 +87,19 @@ public abstract class Patch {
 	 *                {@link Patch#maxPatchVersion(String)}
 	 */
 	protected Patch(@NotNull String modID, String version) {
+		this(modID, version, false);
+	}
+	
+	/**
+	 * Internal Constructor used to create patches that can allways run (no matter what patchlevel a level has)
+	 * @param modID The ID of the Mod
+	 * @param version The mod-version that introduces the patch. When {@Code runAllways} is set, this version will
+	 *                determine the patchlevel that is written to the level
+	 * @param alwaysApply When true, this patch is always active, no matter the patchlevel of the world.
+	 *                    This should be used sparingly and just for patches that apply to level.dat (as they only take
+	 *                    effect when changes are detected). Use {@link ForcedLevelPatch} to instatiate.
+	 */
+	Patch(@NotNull String modID, String version, boolean alwaysApply) {
 		//Patchlevels need to be unique and registered in ascending order
 		if (modID == null || "".equals(modID)) {
 			throw new RuntimeException("[INTERNAL ERROR] Patches need a valid modID!");
@@ -92,6 +110,7 @@ public abstract class Patch {
 		}
 		
 		this.version = version;
+		this.alwaysApply = alwaysApply;
 		this.level = ModUtil.convertModVersion(version);
 		if (!ALL.stream()
 				.filter(p -> p.modID
@@ -185,7 +204,7 @@ public abstract class Patch {
 	}
 
 	/**
-	 * Returns a list of paths,where your mod may IDs in your {@link ru.bclib.api.WorldDataAPI}-File.
+	 * Returns a list of paths where your mod stores IDs in your {@link ru.bclib.api.WorldDataAPI}-File.
 	 * <p>
 	 * {@link DataFixerAPI} will use information from the latest patch that returns a non-null-result. This list is used 
 	 * to automatically fix changed IDs from all active patches (see {@link Patch#getIDReplacements()}