With the help of my professional programmer expert friend, I was able to update my mod as per your basic idea. We basically ended up simply overwriting the vanilla biomes with new definitions. We took the default settings (since we didn't want to change BaseHeight and such) as listed in the public static void registerBiomes() entry in Biome.class (net.minecraft.world.biome), and tinkered with the parts we did want to change. The basic idea, which I'll include here for posterity, is as follows:
(imports to include: import org.apache.logging.log4j.Logger)
public class Main
{
private static Logger logger;
@EventHandler
public void preInit(FMLPreInitializationEvent event)
{
logger = event.getModLog();
MinecraftForge.EVENT_BUS.register(this);
}
@SubscribeEvent
public void registerBiomes(RegistryEvent.Register<Biome> event) {
logger.info("Overriding most biomes for extra snow. There will be some warnings shown:");
/*
* If you want to log all biome names, you can uncomment this:
*
* IForgeRegistry<Biome> registry = event.getRegistry();
* registry.getKeys().forEach(LOGGER::info);
*/
event.getRegistry().register(new BiomeTaiga(BiomeTaiga.Type.NORMAL, (new Biome.BiomeProperties("Taiga")).setBaseHeight(0.2F).setHeightVariation(0.2F).setTemperature(0.0F).setRainfall(0.85F).setSnowEnabled()).setRegistryName("minecraft:taiga"));
}
}
I'll note that when adjusting the vanilla biomes this way, FML throws exception clauses when the game is being initialized, but accepts the changes:
[10:20:54] [main/INFO] [FML]: Potentially Dangerous alternative prefix `minecraft` for name `taiga`, expected `allsnow`. This could be a intended override, but in most cases indicates a broken mod.
Et voila. Updated mod.
https://www.minecraftforum.net/forums/mapping-and-modding-java-edition/minecraft-mods/1288273-1-12-2-forge-sopwiths-allsnow-mod-snow-worlds
Thanks for the suggestive assistance.