Route is AUH-LHR and I'd be buying business and looking to upgrade to first.

Seatspy and Etihad's own website show I could buy the leg in first for 70,000 points and there is availability as a Guest Seat.

Etihad also says the upgrade cost would be 39,750 points.

However, I cannot see how to be certain that I could definitely upgrade that leg before buying the underlying ticket.

Anyone know how to do that online ?