Originally Posted by
ContinentalFan
The question: is easier to get an upgrade if you call and say you'll use miles? Does a mileage upgrade come out of a different, bigger bucket? If so, can I then switch at the airport and use a certificate?
Yes you can switch between certificates and miles any time up to check-in. Certs may be easier for them to process than going to your SkyMiles account to deduct miles, etc. - my guess as to why they automatically ask for certs most of the time.