When you become Wizard, you can Talk to Trebonius, and battle him for the position of Arch-Mage, despite requirements. If you have one brain cell in your head, you'll wait until you become Master Wizard. If you kill him at Wizard level, your status of Arch-Mage will never come to be. You'll just advance to Master Wizard, with no one to promote you to Arch-Mage.