translate(AccountID, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz')
So - this would be another field in your data source, outside the people picker fields. You'd set the default for this field using a formula like the one above, selecting the account id field from the people picker in the formula editor. Does that make sense? You can temporarily put the field on the form so you can verify the formula is working. Then you can compare against that field instead of the account ID.
You could also use the translate function directly when you compare current user to account ID in your rules to determine if the user can approve, but this may be a little easier.