Additionally there is another upgrade at lvl120 to allow you to even level up past 120. The level 100 upgrade quest is optional (but you'd be stupid not to do it). The lvl120 one is a hard cockblock. If you do not complete the quest you will not level higher (but exp is backbuilt for until you DO finally finish it).
Completing the lvl120 upgrade also gives you one free access to change your appearance.