You can roll back the upgrade request, although the ticketing might get a little messy for the agent. From what I understand, they'd have to take you out of X-class for the segments in question, put you back in your original fare class, and then change you to A/C for mileage upgrade.

If there's no availability for your original coach fare class at the time, then it gets even messier. But still possible, IMO.

Keep in mind that getting 2 upgrades expedited might cost more than $75. Don't know from experience, though.