--- a/net/minecraft/util/worldupdate/WorldUpgrader.java
+++ b/net/minecraft/util/worldupdate/WorldUpgrader.java
@@ -38,6 +38,10 @@
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
+// CraftBukkit start
+import net.minecraft.world.level.dimension.WorldDimension;
+// CraftBukkit end
+
 public class WorldUpgrader {
 
     private static final Logger LOGGER = LogManager.getLogger();
@@ -53,7 +57,7 @@
     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 = new ChatMessage("optimizeWorld.stage.counting");
     private static final Pattern REGEX = Pattern.compile("^r\\.(-?[0-9]+)\\.(-?[0-9]+)\\.mca$");
     private final WorldPersistentData overworldDataStorage;
@@ -86,13 +90,13 @@
 
     private void work() {
         this.totalChunks = 0;
-        Builder<ResourceKey<World>, ListIterator<ChunkCoordIntPair>> builder = ImmutableMap.builder();
-        ImmutableSet<ResourceKey<World>> immutableset = this.worldGenSettings.levels();
+        Builder<ResourceKey<WorldDimension>, ListIterator<ChunkCoordIntPair>> builder = ImmutableMap.builder(); // CraftBukkit
+        ImmutableSet<ResourceKey<WorldDimension>> immutableset = ImmutableSet.of(levelStorage.dimensionType); // CraftBukkit
 
         List list;
 
         for (UnmodifiableIterator unmodifiableiterator = immutableset.iterator(); unmodifiableiterator.hasNext(); this.totalChunks += list.size()) {
-            ResourceKey<World> resourcekey = (ResourceKey) unmodifiableiterator.next();
+            ResourceKey<WorldDimension> resourcekey = (ResourceKey) unmodifiableiterator.next(); // CraftBukkit
 
             list = this.getAllChunkPos(resourcekey);
             builder.put(resourcekey, list.listIterator());
@@ -102,18 +106,18 @@
             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
             UnmodifiableIterator unmodifiableiterator1 = immutableset.iterator();
 
             while (unmodifiableiterator1.hasNext()) {
-                ResourceKey<World> resourcekey1 = (ResourceKey) unmodifiableiterator1.next();
-                Path path = this.levelStorage.getDimensionPath(resourcekey1);
+                ResourceKey<WorldDimension> resourcekey1 = (ResourceKey) unmodifiableiterator1.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 = new ChatMessage("optimizeWorld.stage.upgrading");
@@ -125,7 +129,7 @@
                 float f2;
 
                 for (UnmodifiableIterator unmodifiableiterator2 = immutableset.iterator(); unmodifiableiterator2.hasNext(); f1 += f2) {
-                    ResourceKey<World> resourcekey2 = (ResourceKey) unmodifiableiterator2.next();
+                    ResourceKey<WorldDimension> resourcekey2 = (ResourceKey) unmodifiableiterator2.next(); // CraftBukkit
                     ListIterator<ChunkCoordIntPair> listiterator = (ListIterator) immutablemap.get(resourcekey2);
                     IChunkLoader ichunkloader = (IChunkLoader) immutablemap1.get(resourcekey2);
 
@@ -138,10 +142,10 @@
 
                             if (nbttagcompound != null) {
                                 int j = IChunkLoader.getVersion(nbttagcompound);
-                                ChunkGenerator chunkgenerator = ((WorldDimension) this.worldGenSettings.dimensions().get(GeneratorSettings.levelToLevelStem(resourcekey2))).generator();
+                                ChunkGenerator chunkgenerator = ((WorldDimension) this.worldGenSettings.dimensions().get(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)) {
@@ -213,8 +217,8 @@
         }
     }
 
-    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");
@@ -274,7 +278,7 @@
     }
 
     public ImmutableSet<ResourceKey<World>> levels() {
-        return this.worldGenSettings.levels();
+        throw new AssertionError("Unsupported"); // CraftBukkit
     }
 
     public float dimensionProgress(ResourceKey<World> resourcekey) {