If you want to use GUCs, your best plan is to call DL and try to find dates, assuming that you have some flexibility, with upgrade inventory available at the time you book the flights. IIRC you cannot waitlist for upgrades with KLM, nor clear at the gate. If everything works when you're first ticketed, you basically don't need to worry about the fine points of whether codeshares can or cannot be used, ticket stock, etc. It's not a DIY project to use GUCs, so please do NOT purchase this through the KLM website.