You should have asked this in the SAS forum instead as this has nothing to do with UA.
Just because the flight is in codeshare with UA, it does not mean UA rules apply.

Economy extra is a separate class onboard which is sold and marketed as a separate class, hence it is not the same thing as economy plus on UA.
The only way to get there is to pay for it (buy a proper ticket or pay an upgrade fee at the airport) or hope that economy is overbooked so some people are op-uped.
As with all European carriers, SAS does not give complimentary upgrades.