Too much is attached to races to actually do it.
You have different stats, different passives, different systems of racial crafting, racial skills, cosmetic items, and more.
Such a change would either highly unbalance the game, or be useless due to being so devastating to what you've worked on with your character.
Even if the change is in the middle ground and fair somehow, think of how much money just goes into racial skills, those are expensive to get up. Choosing to get rid of them for a new race would be make you lose out on thousands of gold easily, and don't even suggest converting the levels for the skills, because that would just be plainly outright overpowered.