Presuming that your parents were involuntarily denied boarding for their scheduled flight due to an oversell, they are not due any compensation because DL was able to deliver them to their final destination earlier than scheduled.

As you were not present, it is a big mistake for you to handle complaints. They may be elderly, but if they can handle themselves in a routine conversation, we are not talking about an argument in the Supreme Court here. It carries a lot more weight for the agent to hear directly what happened. If it makes you feel better, you can be on the line.

If they make a request (they are the pax, not you, even if you paid), DL will refund the seat fees back to the original form of payment. It doesn't matter whether they were IDB or VDB.

The auto rebook makes no sense, but it is not relevant. DL is required by law to solicit volunteers before involuntarily rebooking and, taking this at face value, that is what happened here. That does not affect your parents, but it does impact DL with DOT.

When your parents return and are home, they should send a short note explaining exactly what happened. DL will likely make some customer service gesture based on the reason for their selection of the connection rather than non-stop.