Success report: booked on Air Canada with mileage plus number. purchased upgrade to P from V via United mobile app a week prior. assigned seat in first. at T-24 offered SDC (in P class with seat in first) through hub city increasing to two segments from one segment. accepted offer of SDC in unted app.
result was that seats were reserved but didnt ticket because stock was not 016. called customer service. got a little push back, but eventually convinced them it was their problem because they made me an offer and I accepted. I am not a lawyer, but generally, that's how contracts are created. anyway, they confirmed me on the new flights and i secured seats in first.