[1.7.2]how can i replace the minecraft class?

[ZYGD]

sorry for my english

i am trying to make a rpg mod and create new movement,so that i was trying to understanding how does the player render work.Now,i know how can i create a new movement.But I have a big problem,how can i replace the minecraft class?(i don't care did my mod support or not support of other mod(Compatibility))

It maybe is a stupid question,hope someone can help me

[ZYGD]

but....how can i make a core mod?