[Paragon]

You ought to be able to do something like what Improved Martial Arts does here; that upgrades the basic martial arts d4 to a d6 with the following code in an eval script:

  if (field[abilActive].value <> 0) then
        perform hero.child[wpUnarmed].setfocus
        perform focus.delete[WeaponDie.2]
        perform focus.assign[WeaponDie.3]
        endif

Assuming the natural weapon is IDed as a wpWeapon too, if you substitute that in and adjust the die sizes as appropriate, it ought to do that job. The only question is what causes it (an edge, hitting a certain level, or whatever)