How can I update existing tickets with approval processes when the approver has changed

You can't I'm afraid.

I am able to override the Approval as a WHD admin. Then I change the request type to one that does not have an approval process attached, and then change it back to the original request type, triggering it to apply the approval process again. I have not had to change an approver yet so I am not sure if the original approver might be "cached" or somehting like that but this has worked for me in the past to make other changes.