When I first looked at signing up, it was online upgrades only, and this is confirmed by the internet archive on 8 March:

https://web.archive.org/web/20230308...ogramme-rules/

This changed at some point during the summer as can be seen on 10 June:

https://web.archive.org/web/20230610...ogramme-rules/

A friend of mine with Skywards + recently travelled ex DXB and the discount could not be applied to his ticket at the airport. In the end he was charged the full amount for the upgrade and waiting for a reply from EK customer service.

I guess that this (positive, for once) change is because the Skywards+ product is clearly not that competitive and has received little interest. Apparently fewer than 10,000 people globally have subscribed...