I would warn you that, as the most likely mechanism for changing the ticket would be cancellation, re-crediting of miles, and rebooking, that making any change to your booked ticket after the date those miles would have expired will most likely result in the immediate "loss" of those miles when the ticket is cancelled as part of the change.

In several other programmes that I know better - Flying Blue being the prime example - this is exactly what happens.

You should play it safe, and consider that the miles are no longer usable in any shape or form after their normal date of expiry. The sole exception being if you have converted them to into an award ticket for future travel prior to their expiry - but that ticket will die, too, if you do anything other than fly it as booked.