Keep in mind that GUC inventory on many DL routes is very limited, so it is best to call DL to see what they have available for GUC upgrades and then plan accordingly. However, my experience is that it often is not possible to get confirmed upgrades for the desired destination and (approximate) date. If you are going to Europe, the most transparent way to go is to investigate KLM-operated flights before calling: If they have availability in Z, then a skilled DL agent typically will be able to make that work for you; it will involve a reissue actually on KLM in Z. This actually works out in your favor from a perspective of MQMs and miles earned on the trip.