Over the last few years, this had been a hit or miss for me to get moved without major discussion. At various airports. Once in HEL, I was on an Go ticket which I upgraded with points to Plus and the agent took a huge effort to get me on the earlier flight as the system said "no" whatever they tried. She just refused to give up and eventually I was moved to the earlier flight. Than I had the same experience like SASegments in CPH and OSL were I was told "no way, unless you have a full flex ticket" on a full bought Plus ticket. The CPH agent eventually gave in, but in OSL... no dice.

I think it was never a real published benefit and you are dependent on the good will of the gate agent. Anyhow, in the last like 5 years, I was refused just two times (once in OSL, once in CDG).