Can only repeat what Ikrt has already wrote. And to add a little to it, bought HEL-SIN-HEL value ticket in Jan for Easter (mid April). The HEL-SIN leg had one available upgrade slot, that I've managed to grab using the online upgrade tool. For The SIN-HEL leg there was no upgrade availability, so I've asked CS to waitlist me. The upgrade cleared ~3 days prior the return flight.
Similar experience with JFK-HEL leg. Stayed waitilisted for few months and upgrade cleared few days before return flight.