I'm not sure I'm parsing this question correctly, but it sounds like a contracts question, and I'm not at all privy to the contract between any airline and Boei

I'm not sure I'm parsing this question correctly, but it sounds like a contracts question, and I'm not at all privy to the contract between any airline and Boeing, whether they have any contingency for this. The actual risk is complex, an airline with more capacity depending on the new model has more risk as they take on new planes and retire old ones, but does the contract reflect that complexity? I have no idea. For all I know, there's generic terms for handling all certification problems that crop up, including airworthiness directives from the FAA that affect A&P's rather than pilots. It might be the two sides split the cost 50/50 for all I know, or maybe they use binding arbitration to keep the airing of dirty laundry as much out of the public view as possible.