From the SAS FAQ:
Originally Posted by https://www.flysas.com/content/dam/sas/pdfs/fly-with-us/plusgrade-faq-en.pdf
IF MY UPGRADE IS CANCELED BY SAS, WHAT COMPENSATION AM I ENTITLED TO AND WHEN WILL I GET IT?
If your confirmed upgrade is canceled by SAS, you’ll receive a full refund of the amount/EuroBonus points you paid for the upgrade within 7 business days. If you paid by credit card, the amount will be credited to your account, and if you paid using EuroBonus points, they will be credited to your EuroBonus account. In the case of a downgrade, no extra compensation will be paid. Your ticket will be returned to its original status in SAS Go or SAS Plus and you’ll be reseated in SAS Go or SAS Plus based on seating availability at the time. Please note that your original seat may no longer be available.
SAS doesn't voluntary pay downgrade compensation in this case and I do not think EU261 downgrade compensation applies, based on the T&Cs of the Plusgrade upgrade process. But you can try...