I've written before about my transition this year from Delta Platinum Medallion to Alaska MVP Gold 75K status. In short, I couldn't justify crediting my paid Delta flights to the SkyMiles program now that they've moved to revenue-based earning and voodoo-based redemption.
My Alaska Mileage Plan elite status means I can freely choose between American- and Delta-operated flights, which is a huge luxury when pricing out paid itineraries.
Given similar-enough prices and convenience, however, I'll typically choose Delta for my paid flights because they run a fantastic airline. But while I earn elite-qualifying and redeemable miles in Alaska's Mileage Plan for those flights, I don't get free Comfort+ seating or free checked bags, and that's not ideal.
For that reason, I also keep Delta Silver Medallion status. Here's how I leverage the two elite statuses.
Don't request Medallion Complimentary Upgrades
If you book a paid reservation through a third-party booking engine like the US Bank Flexperks site and add a SkyMiles number, your Medallion Complimentary Upgrade will be automatically requested.
That's a problem because once a Medallion Complimentary Upgrade has been requested, you won't be able to remove your SkyMiles number from the reservation.
After booking the ticket, you'll need to manually add your SkyMiles number.
If you're buying a ticket directly from Delta, you can simply uncheck the "Request Upgrade" box during booking.
If you did accidentally request an upgrade, you'll need to contact Delta to cancel the request (this is not an unusual request; often couples or business travelers want or need to be seated in the same cabin).
Check in with your SkyMiles number
24 hours before departure, Silver Medallions can request free Comfort+ seats (higher-level Medallions can request them earlier).
If you're checking a bag, you'll also need to drop your bags off at the airport while your SkyMiles number is attached to the reservation in order to avoid checked bag fees.
Remove your SkyMiles number and add your Mileage Plan number
I've had mixed success asking airport agents to remove my SkyMiles number from reservations after check-in. But fortunately, it's simple to do online, as long as you haven't requested an upgrade. Just access your reservation (in the app this is called "Trip Details") and click "Remove" next to your SkyMiles number. Then add your Mileage Plan number. The website will automatically add 3 leading zeroes, but in the app you'll need to type them in yourself, so your Mileage Plan number is a total of 12 digits long.
Reprint your boarding passes
This isn't strictly necessary, but as a precaution I always fly on boarding passes showing my Mileage Plan number and elite status.
Obviously for the sake of convenience it's easier to do this on one-way rather than round-trip reservations, since the more times you have to remove and add frequent flyer numbers, the higher the chance of something going wrong, either on your side on in Delta's reservations system. If I were checking a bag on a roundtrip reservation, I might even consider simply leaving my Mileage Plan number attached to the reservation and paying the checked bag fee, in order to ensure proper crediting to my Mileage Plan account.