As of v150225, Custom Registration/Profile Fields are hidden on Billing Modification Forms. There are many scenarios where it would be desirable to show Custom Registration/Profile Fields on a Billing Modification form, such as when a Free Subscriber is upgrading to a Paid Membership and the paid membership requires some Custom Registration Fields to be filled out.
As of v150225, Custom Registration/Profile Fields are hidden on Billing Modification Forms. There are many scenarios where it would be desirable to show Custom Registration/Profile Fields on a Billing Modification form, such as when a Free Subscriber is upgrading to a Paid Membership and the paid membership requires some Custom Registration Fields to be filled out.