I'd think you'd be charged the new amount. The reasoning is this. You lock in a price (in this case # of miles) only when you TICKET. If you change that ticket it will have to be re-ticketed, which means the price (miles) will be reflective of what is available that day, since that is when the ticket for those flights will be issued. In your case your price would no longer be available and you'd have to move up to what is.