One classmate bought tix by credit card through the site and then later said she couldn't come. She sold her tix to another classmate.

If I delete her registration, it won't affect her payment right? I don't want to refund her because she has already been paid by the other classmate.

So, should I delete her reg, and then add the other classmate to the attendee list?
Or should I just change the names of those attending on her registration to the new classmate and her guest?

You can delete her registration so that Reports and Nametags will be right. Then I would go to the Reunion Planner, Attendees page, scroll down to the bottom, then choose ADD and add the new classmate, Register the classmate, purchase a ticket, say "CHECK" or "OTHER" for the payment type. On the DISCOUNT, mark it as "100" so that their ticket is free. Then save the attendee.

Thank you Kyle!