I went through a similar situation earlier this year on an award booking. When I called KE reservations in the USA, the agent was very helpful. He was able to book the new itinerary, even though I did not have the miles. He was able to hold it for 2 days. Then I cancelled the original booking and waited for the miles to come back to my account. They said it would take 2 days but it actually happened in a matter of hours. I was then able to confirm the revised itinerary.