Huge mistake. Yesterday the seat map changed back to the original Airbus A350 with Premium Economy in 2‐4-2 config. Obviously 15K is out the window as it doesn't exist on this aircraft. Now today, date of departure, it's changed AGAIN to the A350 without a separate PE cabin. Economy is in an all 3-3-3 config. I've been auto assigned 19A instead. I knew I should have waited.
I am not a frequent VN flyer, how often are aircraft switched/downguaged on this route?
To answer my own question: Quite often it appears!
Edited to add: Final aircraft swap at the airport, seat has been auto changed again, the final seat configuration is now a Boeing 787 all Economy class configuration in 3-3-3. Lovely 🙄