You can turn in a JR pass voucher ("exchange order") for a refund minus a 10% handling fee but only if you've not activated it. If your plans change and you have to leave Japan early for some reason an activated pass voucher is "used up" even if it was for a later period, and you can't get a refund on it.
It doesn't take long to activate a pass when you're in a major station or at an airport so I'd suggest you only activate one at a time, just in case. It also gives you a bit more flexibility in your travels if you change your mind during your stay on when you plan to travel.
One important thing to watch out for if you do this though is that there's a time limit on using an exchange order after buying it. Condition 2 on the exchange order says it must be exchanged for a JR pass within three months from the date of purchase, which is stamped on the pass when it is sent to you by the seller.
It looks you're planning to be in Japan for over two months according to your sample itinerary. If you buy your exchange orders too far in advance the later orders might have expired when you come to exchange them. For example, if you had purchased several passes on February 17th you'd have to activate the last one before May 17th otherwise it would expire.