Just made a successful voucher upgrade. Booked ARN-IST-EZE round trip in Economy Class with the quickest connections. Made a pay&fly booking and gave the Elite Plus line a call to confirm the upgrade availability. I had previously checked the availability on ExpertFlyer, and J-class was available on the selected flights. Interesting to note is that J-class is no longer sold as paid tickets from Scandinavia, so you have to check twice before completing any ticketing and upgrade transactions.
Once the ticket was issued through the mobile app (the website didn't accept my Amex for whatever reason), I instantly gave them a callback and completed the upgrade on the phone with a different agent. The upgrade process took roughly 15 minutes to complete, and the agent was very professional. All four segments upgraded to Business Class with both of my vouchers which is an excellent deal.
Despite the somewhat high fares in Economy Class, this benefit definitely makes it worth wile paying the extra premium. I paid €800 for a round trip ticket in Economy Class over the Christmas and New Year's period which is an okay deal.