Oh okay, now I found it. You are on the website, because the app does not have a "My bookings" section.

Indeed, now I have one flight I can upgrade with miles or evouchers. It's for 4 people and it would let me use 4 evouchers, so it seems to work well although I did not actually book it.
But weird I don't see it in the world's best airline app. And I can also not upgrade this specific flight with evouchers in the app so it seems to be a website-only function.

Good to know though. If it works well, it really is an improvement. Thanks again!