You must go to the Shore Excursions desk once onboard to swap the credit card shore excursion booking for one using non-fundable OBC. (Guest Services cannot handle this.) Do not ask them to cancel and re-book. The amount will be deposited as refundable OBC to your account. If you haven't used it by the end of the cruise, the refundable OBC will credited to the credit card you have on file for the cruise - not necessarily the one used to book the excursion.
I am not sure if the thermal pass uses the same procedure.
Quite a few cruisers have reported being able to do this for shore excursions.