In Hall of Changes you have 2 options:
1. Visit the Hall of Changes
2. Ask about changing race
First option, visit, allows to change pet's and character appearance. Just click on character or pet symbol on left.
Second option, changing race allows to change character's race to another available for your class. Changing race to Aoidos is available only if you have created Aoidos incarnation with item you got at 70 or 75 level via ingame mail. Creating incarnation is possible after you reach 80 level and do reincarnation quest chain though. You need also to level up this incarnation to max level.
Both options require carnival masks from boutique.