Well first I would suggest, instead of trying to change a default, grab my milkshape tutorial and grab the tutorial it speaks of from
http://www.modthesims2.com (if it is still available) and learn how to extract and modify a mesh successfully, once you can do that without any problems, then you can proceed to making a new default mesh.
Edit:
I just checked, the tutorial is still available from
http://www.modthesims2.com through the link in my tutorial...BriAnna's tutorial will teach you about extracting the mesh and putting the mesh back into the game, my tutorial will help you learn milkshape which is one of a few programs you can use to edit the meshes from the game.