You can change anything on a Mech except its Weapon Systems.
Ammo, Equipment, Armor Distribution, and Uprades can be customized.
This rule gives us "semi-stock" mechs that we can all validate when targeting our opponents.
There was a poll a while ago where a 70% of the people who responded of people expressed interest in a stock mech only game mode (http://mwomercs.com/...only-game-mode/), but PGI doesn't have this in their road map at this time.
Edited by TheForce, 09 April 2013 - 10:18 AM.