Originally Posted by
Wharfe
Paid seat is okay for upgrade, but Eithad may ask for several 100k miles instead of what the calculator says, regardless if a GuestSeat is available in the higher class (This is from personal experience).
They may also offer the calculator price, but it seems hit or miss. And you'll not know until after you've paid for the flight and then call the help desk asking for upgrade using miles.
So the "game theory" should be either:
a) book the ticket on the phone with the agent who gave you the upgrade cost and make it a combined deal
or
b) get the agent on the phone to give the upgrade points cost and you have your booking fulfilled only to press "order" and that generates your booking code