Occasionally, PMUA will offer the buy-up option when booking your reservation... Immediately after entering your payment info, the offer comes. Every time I've taken this offer, it books into a high-yield fare class (for that segment only). This is completely different from using the new "Premium Seating" option that appears under the Travel Options section of your reservation. This will only upgrade; no change in fare class.

AFAIK, the booking-time buy-up still happens if space is available.