Changing a role from Observer to Reviewer or Non-Mandatory to Mandatory or vice versa shouldn't necessarily wipe out one's approval.
It may be appropriate to reset the approval if the meaning of approval is interpreted differently, according to the role, but sometimes a person might give a thorough and knowledgeable review who had been assigned a lesser role in the review because of their expected unavailability. Then, if they turn out to participate well, it might be appropriate to update their role in the review, without needing to wipe out their approval. Currently, this isn't possible.