I am not a JetBlue flyer, and there may be some other strategy, but I would simply call customer service and explain that I will be not be flying B6 between now and the end of the year, but I will be flying [insert airline], and ask if it would be possible to change my airline preference. If the initial answer is no, I would speak with a supervisor and politely mention that competing cards issue by Citi and Chase do not limit reimbursements to a single airline.