Data point:
eVoucher upgrade from the cheapest Premium Economy N to Business on OS VIE-BKK, flight in July 2024.
Upgrade requested on Dec 16 2023 and put on a waiting list.
Upgrade all of a sudden cleared on April 27 2024.
For reference, instant upgrade was offered at 2,700 EUR (!) and minimum bidding at 550 EUR.
I looked at the seat map after upgrade, Business Class cabin seats 50% occupied, including 1K. Luckily, throne seat 1A available