Originally Posted by
Wasabi Tofu
Is this award or revenue ?
If revenue, an ANA's agent is wrong.
You can hold part of your itinerary, change your route, then recalculate and reissue tickets.
I am quite sure it is an award booking, it it was a revenue booking, it would be neither here nor there whether OP had mileage in the account to make the change from J to F.