--- a/net/minecraft/util/worldupdate/WorldUpgrader.java
+++ b/net/minecraft/util/worldupdate/WorldUpgrader.java
@@ -48,7 +48,7 @@
     private static final Logger LOGGER = LogUtils.getLogger();
     private static final ThreadFactory THREAD_FACTORY = (new ThreadFactoryBuilder()).setDaemon(true).build();
     private final IRegistry<WorldDimension> dimensions;
-    private final Set<ResourceKey<World>> levels;
+    private final Set<ResourceKey<WorldDimension>> levels; // CraftBukkit
     private final boolean eraseCache;
     private final Convertable.ConversionSession levelStorage;
     private final Thread thread;
@@ -59,14 +59,14 @@
     private volatile int totalChunks;
     private volatile int converted;
     private volatile int skipped;
-    private final Object2FloatMap<ResourceKey<World>> progressMap = Object2FloatMaps.synchronize(new Object2FloatOpenCustomHashMap(SystemUtils.identityStrategy()));
+    private final Object2FloatMap<ResourceKey<WorldDimension>> progressMap = Object2FloatMaps.synchronize(new Object2FloatOpenCustomHashMap(SystemUtils.identityStrategy())); // CraftBukkit
     private volatile IChatBaseComponent status = IChatBaseComponent.translatable("optimizeWorld.stage.counting");
     private static final Pattern REGEX = Pattern.compile("^r\\.(-?[0-9]+)\\.(-?[0-9]+)\\.mca$");
     private final WorldPersistentData overworldDataStorage;
 
     public WorldUpgrader(Convertable.ConversionSession convertable_conversionsession, DataFixer datafixer, IRegistry<WorldDimension> iregistry, boolean flag) {
         this.dimensions = iregistry;
-        this.levels = (Set) iregistry.registryKeySet().stream().map(Registries::levelStemToLevel).collect(Collectors.toUnmodifiableSet());
+        this.levels = (Set) iregistry.registryKeySet().stream().collect(Collectors.toUnmodifiableSet()); // CraftBukkit
         this.eraseCache = flag;
         this.dataFixer = datafixer;
         this.levelStorage = convertable_conversionsession;
@@ -93,12 +93,12 @@
 
     private void work() {
         this.totalChunks = 0;
-        Builder<ResourceKey<World>, ListIterator<ChunkCoordIntPair>> builder = ImmutableMap.builder();
+        Builder<ResourceKey<WorldDimension>, ListIterator<ChunkCoordIntPair>> builder = ImmutableMap.builder(); // CraftBukkit
 
         List list;
 
         for (Iterator iterator = this.levels.iterator(); iterator.hasNext(); this.totalChunks += list.size()) {
-            ResourceKey<World> resourcekey = (ResourceKey) iterator.next();
+            ResourceKey<WorldDimension> resourcekey = (ResourceKey) iterator.next(); // CraftBukkit
 
             list = this.getAllChunkPos(resourcekey);
             builder.put(resourcekey, list.listIterator());
@@ -108,18 +108,18 @@
             this.finished = true;
         } else {
             float f = (float) this.totalChunks;
-            ImmutableMap<ResourceKey<World>, ListIterator<ChunkCoordIntPair>> immutablemap = builder.build();
-            Builder<ResourceKey<World>, IChunkLoader> builder1 = ImmutableMap.builder();
+            ImmutableMap<ResourceKey<WorldDimension>, ListIterator<ChunkCoordIntPair>> immutablemap = builder.build(); // CraftBukkit
+            Builder<ResourceKey<WorldDimension>, IChunkLoader> builder1 = ImmutableMap.builder(); // CraftBukkit
             Iterator iterator1 = this.levels.iterator();
 
             while (iterator1.hasNext()) {
-                ResourceKey<World> resourcekey1 = (ResourceKey) iterator1.next();
-                Path path = this.levelStorage.getDimensionPath(resourcekey1);
+                ResourceKey<WorldDimension> resourcekey1 = (ResourceKey) iterator1.next(); // CraftBukkit
+                Path path = this.levelStorage.getDimensionPath((ResourceKey) null); // CraftBukkit
 
                 builder1.put(resourcekey1, new IChunkLoader(path.resolve("region"), this.dataFixer, true));
             }
 
-            ImmutableMap<ResourceKey<World>, IChunkLoader> immutablemap1 = builder1.build();
+            ImmutableMap<ResourceKey<WorldDimension>, IChunkLoader> immutablemap1 = builder1.build(); // CraftBukkit
             long i = SystemUtils.getMillis();
 
             this.status = IChatBaseComponent.translatable("optimizeWorld.stage.upgrading");
@@ -131,7 +131,7 @@
                 float f2;
 
                 for (Iterator iterator2 = this.levels.iterator(); iterator2.hasNext(); f1 += f2) {
-                    ResourceKey<World> resourcekey2 = (ResourceKey) iterator2.next();
+                    ResourceKey<WorldDimension> resourcekey2 = (ResourceKey) iterator2.next(); // CraftBukkit
                     ListIterator<ChunkCoordIntPair> listiterator = (ListIterator) immutablemap.get(resourcekey2);
                     IChunkLoader ichunkloader = (IChunkLoader) immutablemap1.get(resourcekey2);
 
@@ -144,10 +144,10 @@
 
                             if (nbttagcompound != null) {
                                 int j = IChunkLoader.getVersion(nbttagcompound);
-                                ChunkGenerator chunkgenerator = ((WorldDimension) this.dimensions.getOrThrow(Registries.levelToLevelStem(resourcekey2))).generator();
+                                ChunkGenerator chunkgenerator = ((WorldDimension) this.dimensions.getOrThrow(resourcekey2)).generator(); // CraftBukkit
                                 NBTTagCompound nbttagcompound1 = ichunkloader.upgradeChunkTag(resourcekey2, () -> {
                                     return this.overworldDataStorage;
-                                }, nbttagcompound, chunkgenerator.getTypeNameForDataFixer());
+                                }, nbttagcompound, chunkgenerator.getTypeNameForDataFixer(), chunkcoordintpair, null); // CraftBukkit
                                 ChunkCoordIntPair chunkcoordintpair1 = new ChunkCoordIntPair(nbttagcompound1.getInt("xPos"), nbttagcompound1.getInt("zPos"));
 
                                 if (!chunkcoordintpair1.equals(chunkcoordintpair)) {
@@ -227,8 +227,8 @@
         }
     }
 
-    private List<ChunkCoordIntPair> getAllChunkPos(ResourceKey<World> resourcekey) {
-        File file = this.levelStorage.getDimensionPath(resourcekey).toFile();
+    private List<ChunkCoordIntPair> getAllChunkPos(ResourceKey<WorldDimension> resourcekey) { // CraftBukkit
+        File file = this.levelStorage.getDimensionPath((ResourceKey) null).toFile(); // CraftBukkit
         File file1 = new File(file, "region");
         File[] afile = file1.listFiles((file2, s) -> {
             return s.endsWith(".mca");
@@ -288,7 +288,7 @@
     }
 
     public Set<ResourceKey<World>> levels() {
-        return this.levels;
+        throw new AssertionError("Unsupported"); // CraftBukkit
     }
 
     public float dimensionProgress(ResourceKey<World> resourcekey) {