This reply is a little late considering you posted back in February. My experience was: I made my reservation first, then applied my KTN to my SWA profile the night before I logged on to do the online check-in, 24 hours prior to my flight. My KTN was not printed on the boarding pass. I immediately started searching in the Discussion Forum to find out why my KTN was not on the boarding pass. I thought it would populate since it was saved on my profile. I found instructions on how to add your KTN to your existing reservation, but you can't do it if you've already checked in. You MUST call the customer service number to have them add it to your reservation. You could probably have them do it at the airport, but I didn't want to waste any extra time doing extra things. The rep asked for my confirmation number and had me re-check in. I did not lose my assigned boarding position. The words "TSA PRE" were now above my name on my boarding pass. Apparently, early morning is not a good time to call, they are busy, so I opted to have them call me back and not lose my spot in the queue. Took them over an hour to call me back to ask my question, but it was a quick 2-minute phone conversation and resolution. Good job SW!
