However, in the past (pre-COVID) we learned that there are 2 types of OBC: refundable and non-refundable. Promotional OBC is usually non-refundable, but OBC from your TA or credit card may be refundable. Non-refundable OBC is used first, and if there is leftover of refundable OBC, you will get a refund heck from Celebrity in the mail a few weeks after the cruise.
Now it's possible that policies may have changed post-COVID... but you could ask at Guest Services whether any of your OB is refundable.