Hi all!

I am looking to book five tickets to Europe next summer; multi-city in main cabin. Delta's website requires me to book the two one-ways tickets separately, despite both outgoing and inbound flights being Delta operated. Using Google Flights, I can book on KLM's website at a $400\ticket lower cost (both equivalent to main cabin, not basic). We do have 8 GUCs we can utilize and in prior years we received 'pay x amount to upgrade your flight offers', which we've always taken in the past. If we book through KLM's website (aside from the difference in MQD earning), will this impact our ability to combine GUC\$ to upgrade from main cabin to Delta One on outbound (inbound is only Premium Select, as it's a smaller plane) and are there other factors to consider? Our family is two diamonds and three gold medallions (earned, not gifted if that matters). Thanks all in advance.