If you generally fly J then Silver status has pretty much zero additional benefit...

I think people misread the OP.

The answer is: if on 31/10 your first flight hasn't hit your account then call MPC and explain the situation. They will probably be able to credit it straight away. On 1/11 your new membership year will start and the second flight will definitely credit to it.

If I've misunderstood and you if fact for some reason want to revert to Green and then credit both flights to the next year then the answer is to get your MPC number out of your reservation and then manually claim the credit once you have dropped to Green.