Depending on your ticketing deadline, you may be able to book first and wait for the transfer to be done to ticket.

In my earlier post, I am neutral because:
(1) If you cannot use the Asia Miles, you may end up losing money.
(2) If you change your itinerary, you may not be able to find another date that have U availability. You are stuck with that date!
(3) Besides, some business class fares, such as DONEx, and TPE-HKG-LAX/SFO roundtrip when there is promotion, have better values - they are much less restrictive, and not cost much much more. (This is not applicable in your case).

In my case, I have plenty of Asia Miles and SPG points, so I have no plan of buying any points or transferring any to Asia Miles.