I'm trying to use GameRegistry.addSubstitutionAlias:
GameRegistry.addSubstitutionAlias("minecraft:fire", GameRegistry.Type.BLOCK, (new moddedfire()).setHardness(0.0F).setLightLevel(0.0F).setBlockName("fire").setBlockTextureName("fire"));
I intentionally set the light level to 0, so that it would be obvious if it worked. I can't seem to get it to work though. It doesn't crash, but any world I load (even a brand new one) tells me I'm using a backup level.dat when I try to open it, and asks if I want to continue. When I hit yes, the world loads normally, but the fire still gives off light.