Color me doubtful. The production dates and cast were announced in the spring of 2008, and planned well before that. You can see Ning Liang's planned opera performance schedule in OperaBase. She has a good chunk of time alloted for The Letter.
Late withdrawals happen for a number of reasons:
- Illness, such as the late Lorraine Hunt Lieberson's withdrawal from Doctor Atomic
- Change in the role's requirements, per this press release about how Richard Paul Fink replaced Friedemann Roehlig in Doctor Atomic (Reading that press release now, one wonders about it.)
- Unprepared artist. A tenor turned up in San Francisco a few years ago for Don Carlo without having learned the part in French. Hello? Didn't you and your agent read the contract?
- Artistic differences/singer doesn't get along with director/singer hates role requirements/director doesn't like singer.
A scheduling conflict is hard to swallow. Opera contracts are negotiated years in advance, and while I've heard of singers withdrawing from one production because of a great opportunity elsewhere, what's going on that is more important than the premiere of a new opera?