Can a motor requiring 91-octane fuel be reprogramed to us a lower octane rating of 89 or 87 octane?
A modern car is already capable of sensing a lower octane fuel and it can adjust the ignition timing to accommodate the cheaper gas. There are limits, however. A high compression engine will not thrive on 87-octane fuel.
A spacer between the cylinder head and the engine block would work. Pistons with lower tops would work, too. A milder cam profile could help. Retarded timing is another way.
in other words, the cost wouldn’t be worth it