If not sold yes, should be available for upgrades but the flights close ~ 3hrs prior to departure when check-in opens. Some people have had upgrades clear at the gate so it could happen any time.
Unless you can confirm them prior it’s a fingers crossed situation because you don’t know who else is in the upgrade pool with you.
I think paying for a twin seat long haul is worth it for the extra space. I lift the arm rest up and swing my legs into the middle seat area which is good for a stretch. I don’t think they make you pay though if it’s light loads.