Dave is correct, there was not much difference between the lowest fare and the K fare that I purchased.

I called tonight and was able to get everything changed. It took some time because the first agent booked M fare tickets. I think that she thought that Mexico required M or higher like going to Europe does. The second agent I talked to also thought that I had to have an M fare or better. I had her look at the terms and she cancelled the 2nd set of tickets that were booked with the M fare and started over. I also had to go online and find two of the SWU certificate numbers for her to apply.

All and all it was not to bad. Plus now I get another day in Cancun.

My wife and I have been upgraded before on this route but since I had the SWUs and the ticket price for K was not that much different, I just wanted to lock the upgrade in. I have been buying F tickets for my domestic trips and I was able to use the buy one get one free Amex Reserve certificate to get 2 F tickets to LGA for $645 earlier this month. So, the SWU are better served here.

Thanks to everyone for their advice.