I just booked my 1st Mint experience too. I turn 40 this week and decided to treat myself. I had booked a RT JFK/LAX/LGB/JFK and it was a pain to upgrade to Mint over the phone. The agent basically cancelled the original outbound leg and rebooked it as a separate reservation with Mint rather than simply upgrading the seat. I thought that was odd.

With about two weeks to go until departure this July, and what appeared to be four or five sold Mint seats, the one-way fare to the West coast dropped to $649. I upgraded my Economy seat on the JetBlue website, paying the applicable fare difference. Based on my observations over the past few days, they are willing to sell three or four seats at the $649 rate if the number of seats sold is four to five or lower.

After completing the transaction, including selecting seats, the website advised that the payment went through, but that the ticket needed to be reissued. The seat I had selected continued to show as available for fresh bookings, so I eventually called B6 to inquire. The phone agent managed to grab my original seat, but it was never clear whether this required her to reissue the ticket. Her view was that the website could be a little buggy for these things and that the upgrade is better handled over the phone, but the experience of the prior poster suggests that phone, too, has its challenges.

I gathered that unless booked initially as Mint, the whole itinerary has to be changed and rebooked with the Mint added. The tickets are reissued as well.

Not an ideal system.

There should be no change fee for anyone to up-fare to Mint, regardless of status.

This is correct, it's an upgrade not a rebooking (although they do rebook the tickets in their system).