These are the exact rules:
Change fees and/or additional fare collection will be waived if the rescheduled flights are booked in the original class of service to the same city pairs as originally ticketed and rescheduled travel commences on or before March 23, 2007.
If the new travel commences after March 23, 2007, change fees will be waived, however, additional fare collection will apply.
I failed to mention that I changed from MCO to TPA. I'm not sure if that is the reason I would have been charged for the Y flight. But the OP didn't change cities.