DL has eliminated the availability of Z class on DL marketed/AF operated flights since December. Z was the fare class used for Skymiles upgrades. This was done without advance notice to AF or Skymiles members. The passengers are accepted in economy as that is what DL remits to AF since suppressing the miles upgrades.

Skymiles awards are booked in O (business) and X (economy) under the AF flight numbers and are not affected.

There have been some posts about this but couldn't locate anything with a quick search.