Is there any way to display these mandatory required fields also in a upgrade workflow?
Not currently, no. We've plans to implement some way of doing this in a future release. For now the incubator project CB Progress with CB Triggers will allow force redirect to profile edit if the user has incomplete required fields.
Is there any way to respect admin approval also for upgrade process?
No, approval and confirmation only applies to registration workflows. Suppose only way to force approval is use offline payment gateway.