How to replace a Minecraft base class?

[Dimaspy]

I have edited the class WorldServer (jr.class) and am trying to replace the current one in the Minecraft.jar file.

 

In the vanilla version I can do this easily, by removing the META-INF, but Forge crashes.

 

So how do you edit Minecraft base/core files without having Forge go hardcore paranoid?

[GotoLink]

Coremod.

[Dimaspy]

Implying a single word will help me solve my problem.

 

At least, mind providing a link?

[GotoLink]

http://www.minecraftforge.net/wiki/Using_Access_Transformers

[Dimaspy]

Thank you.