Many of these replies don't match my own experience, which is that if you book and waitlist there is no change fee if your waitlist preference comes up, nor do you need enough miles to cover both your initial confirmed booking AND your waitlist. However, you should speak to Asia Miles / Marco Polo to arrange this.

Your status will definitely make a difference. As CX Diamond I get my preferred flights (eventually) most (but not all) of the time.

Good luck!!