In the FAQs it states:
Originally Posted by
vysean
I'm booked in [First, Business] but one of my flights is in [Business, Coach] - can I change it to the higher class?
Yes, before travel commences, you can change the class of service on a segment if a seat becomes available. There should be no cost to do this. Note that you must have paid the higher mileage rate when you originally ticketed the award.
What if I book an award ticket in Business (because, at the time of booking, there is NO First Class saver available). However, before travel commences, First Class saver becomes available. Is it possible to re-book into First Class?
And, if so, what fees (if any) will be associated with this change?