For WCA web forms configuration, offer a checkbox to allow the web form builder to enable (or not) IBM ID processing based on email address. After user enters their email address into a web form field, check (ajax?) whether they already have an IBM ID, and pre-populate any of that WCA web form's fields that already have known values (from their IBM registration). Include an informational message that the fields have been pre-populated with values from the contact's IBM registration record. If the checkbox isn't checked, no need to incorporate this processing.