A simple class to change the dimension type of a world. For Spigot and Minecraft version 1.18.1.

CustomDimension.java

/*
 * MIT License
 *
 * Copyright (c) 2022 Marvin (DerFrZocker)
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */
package de.derfrzocker.testplugin.customdimension;

import net.minecraft.resources.ResourceLocation;
import net.minecraft.server.level.ServerLevel;
import net.minecraft.world.level.Level;
import net.minecraft.world.level.dimension.DimensionType;
import org.bukkit.craftbukkit.v1_18_R1.CraftWorld;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.world.WorldInitEvent;

import java.lang.reflect.Field;
import java.util.OptionalLong;

public class CustomDimension implements Listener {

    private final static Field FIXED_TIME_FIELD;
    private final static Field ININIBURN_FIELD;
    private final static Field AMBIENT_LIGHT_FILED;
    private final static Field DIMENSION_TYPE_FIELD;

    static {
        try {
            FIXED_TIME_FIELD = DimensionType.class.getDeclaredField("w");
            FIXED_TIME_FIELD.setAccessible(true);
            ININIBURN_FIELD = DimensionType.class.getDeclaredField("K");
            ININIBURN_FIELD.setAccessible(true);
            AMBIENT_LIGHT_FILED = DimensionType.class.getDeclaredField("M");
            AMBIENT_LIGHT_FILED.setAccessible(true);
            DIMENSION_TYPE_FIELD = Level.class.getDeclaredField("C");
            DIMENSION_TYPE_FIELD.setAccessible(true);
        } catch (NoSuchFieldException e) {
            throw new RuntimeException(e);
        }
    }

    @EventHandler
    public void onWorldInit(WorldInitEvent event) {
        ServerLevel level = ((CraftWorld) event.getWorld()).getHandle();
        try {
            DIMENSION_TYPE_FIELD.set(level, createCustomDimension(level.dimensionType()));
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }

    private DimensionType createCustomDimension(DimensionType original) {
        return DimensionType.create(
                getFixedTime(original), // fixedTime -> w
                original.hasSkyLight(), // hasSkyLight -> x
                original.hasCeiling(), // hasCeiling -> y
                original.ultraWarm(), // ultraWarm -> z
                original.natural(), // natural -> A
                original.coordinateScale(), // coordinateScale -> B
                original.createDragonFight(), // createDragonFight -> C
                original.piglinSafe(), // piglinSafe -> D
                original.bedWorks(), // bedWorks -> E
                original.respawnAnchorWorks(), // respawnAnchorWorks -> F
                original.hasRaids(), // hasRaids -> G
                original.minY(), // minY -> H
                original.height(), // height -> I
                original.logicalHeight(), // logicalHeight -> J
                getInfiniburn(original), // infiniburn -> K
                original.effectsLocation(), // effectsLocation -> L
                getAmbientLight(original) // ambientLight -> M
        );
    }

    private OptionalLong getFixedTime(DimensionType original) {
        try {
            return (OptionalLong) FIXED_TIME_FIELD.get(original);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }

    private ResourceLocation getInfiniburn(DimensionType original) {
        try {
            return (ResourceLocation) ININIBURN_FIELD.get(original);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }

    private float getAmbientLight(DimensionType original) {
        try {
            return (float) AMBIENT_LIGHT_FILED.get(original);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }
}