A couple of classmates registered for the reunion and indicated that would send a check, now they have decided they want to pay via credit card. Can they change this?

Yes. First:

1) Click Reunion Planner
2) Click the Show Attendees link
3) Click directly on the Classmates' names
4) Remove their orders (just be sure you only delete the items ordered, don't delete their entire registration)
5) Tell the Classmates to click the Reunion Planner link again. Then add their reunion tickets to the cart again, and this time check out with a credit card.