Backup JR pass/return policy tactics

Hello all,

Im planning a big trip in September in which I fly in and out of Tokyo (7 weeks apart). Ive currently got a 4 week travel plan within Japan but after that is likely going to be a mystery to me until that time comes. I might fly out to another nearby country, or choose to stay within Japan.

So to get to the point I know I'll need 4 weeks of JR pass but theoretically could need up to 7. From the T&Cs I see I can send of postal return of an exchange pass for an 85% refund (i think) or exchange a JR pass in person in Japan for a 90% refund (as long as I do it before the start date)

Is this correct? Is it that simple? Seems like a brilliant solution to my problem.

Thanks for any info :)