AFAIK, you cant. The moment you cancel the original booking to in order to make the 'switch', the expired miles will disappear.

How many of your miles were expiring? If only part of the miles used for the upgrade were expiring, you could still save your 'expired miles' by changing only the legs that were upgraded using the 'non expired' miles. The system utilizes the miles on a FIFO method, so if you upgraded the onwards first, your 'expiring' miles would have been utilized there. So, if you make changes to your return which was upgraded using the 'non expiring' miles, you should be fine and will not lose any miles.