Related to the above...
We did that (previous post) when we got a DING saying our outgoing flight was cheaper. We later got a DING saying our return flight was cheaper, but we didn't change it b/c other flights for that day were unavailable. We were afraid that if we cancelled to rebook that we might miss the window and someone else would fill our seats. Did we make the right choice?
If you get all the way to the page where you just have to insert the payment and do it quickly (i.e. don't let the page expire), those tickets are yours...
So if you insert all NEW ticket information, open new browser, insert cancellation information, hit cancel, and hit "purchase tickets" on Ding window, your seats are just flipped over from one price to the other... I don't see how someone else would be able to fill your seats like that.
For this trip, I had booked two of us on one reservation so we got sequential boarding passes. When a Ding came out, if I typed in "2" for number of passengers, it came up as unavailable. If I typed in "1" for number of passengers, it was fine.
Well, I opened, 3 windows and worked some magic!
1) had my ticket
2) had DBFs ticket
3) cancelled old ticket
I did it in the order above and now have 2 different reservations for the flight going down but we will survive.
If you do it the order I suggested (get to the payment page and insert ALL information except CC # and exp. date) and then cancel, you are set to go.