bgriff

Successfully used GUCs on KLM for an NYC-CPT roundtrip, after a lot of work. Some findings:

• Agents were very easily confused about the rules, and several started off with "those need to be booked in the higher fare buckets." They did all go in and look up the rules, and some corrected themselves, or agreed once I asked them "but not if it's DL-marketed right?"
• Using ExpertFlyer I couldn't find Z space around my dates (next April/May) on the NYC-AMS-CPT routes at all -- not clear if it hasn't opened up yet or what. And even on other dates (such as Feb-Mar), the KL-operated JFK-AMS flights tended not to have Z space, though supposedly some DL-operated flights to AMS do, in combination with the KL flight to CPT, which I'm suspicious how "real" that data from EF is. But for my dates even that wasn't available.
• But ORD-AMS-CPT was readily available, so I targeted that routing. And I was able to use ITA Matrix to find a JFK-ORD-AMS-CPT and back routing, and was able to book it using Book With Matrix. Fortunately the agents could still see the ORD-CPT upgrade space so not sure if adding domestic Delta legs doesn't affect KLM's married segment upgrade availability, or if I just got lucky that it was still available. (EF won't show availability for the NYC-ORD-AMS-CPT itinerary specifically.)
• One agent I started talking to was constructing the itinerary and put it on hold for me since I hadn't requested the GUCs yet at that point. I could see online that he was building it with KLM flight numbers instead of DL flight numbers. I think he might have been skipping ahead to the "re-code the flights onto KLM flight numbers" step that is required when processing the upgrade, but he was also quoting me the (higher) price that resulted from the KLM flight numbers, so I didn't end up using the itinerary he'd assembled. But at least he was able to confirm that adding the JFK leg didn't seem to affect Z availability from ORD onward, so that gave me the comfort to go ahead and request the GUCs.
• Based on that experience if you are in a case where the KLM flight numbers are cheaper, it might be worth trying to have the agent check for space and build the itinerary all at once to see if they accidentally quote you the KLM price. If the DL-marketed flights are cheaper, may be better to book yourself and then call to have them process.
• I had considered booking JFK-ORD as a separate ticket, but there was more irrops risk that way, and it turns out it would have been more expensive, so glad I could get it all on one ticket. Definitely worth playing around with ITA Matrix, especially because even if you do book with an agent, pricing can jump up or down randomly depending which connecting flight you choose, and ITA can show you those options much more easily than agents seem to be able to look them up.
• In general, non-Delta-hub cities seemed to have more KLM availability -- ORD, IAD and SFO all had better availability through to CPT than JFK and ATL did, at least for my dates. So if you don't mind the extra segment and can price it out without too much extra expense, routing through those other cities may be the way to go. Of course this only works well if you are already based in a Delta hub and so have a nonstop Delta flight available to a city like ORD; if you have to connect through ATL to get to ORD it may not be worth it, and from looking on ITA Matrix it looks like that can cause the prices to get much more expensive anyway. If you were based somewhere like RDU for example you could consider buying a separate nonstop ticket on AA or UA to get to ORD and then booking an upgraded DL ticket just on ORD-CPT, but if you were going to do that make sure you leave yourself plenty of connecting time in ORD (personally I'd probably plan for an overnight in Chicago on the outbound flight) to account for the possibility of delays and cancellations.
• Once I had requested the GUCs and figured out this random issue, called back, by chance got a great agent and steered her directly to the all-DL-marketed booking I'd made through Book With Matrix (by confirmation number so as not to revisit the more expensive held booking the earlier agent had set up for me). I could tell she would be good because she immediately started talking about how she was looking up and following the instructions on their internal reference system -- some of the other agents had seemed to be winging it, which doesn't seem to work, especially because most such agents seem to think they use the normal DL process, and KL is different.
• After 45+ minutes of work on her part, much of it with me on hold while she talked to another desk, and that other person talked to their supervisor, the whole thing was eventually confirmed. As expected the JFK-ORD hops are in RY while the KLM flights were re-coded from the DL flight numbers to the KL flight numbers and ticketed in Z.

All in it was about a 4 or 5 hour process to speak to several different agents, not to mention having spent weeks beforehand playing with availability on EF and routings on ITA, and months beforehand keeping up with the various threads of others' experience on FT. So it is possible to make good use of the GUCs on KLM, but it definitely isn't easy.