Originally Posted by Stripe

It makes no sense to be "confirmed and protected" on a single flight. Most airlines can block space for you on a later flight, but not take you off of your original flight, if it looks like you might misconnect. If you do misconnect, an agent can then confirm you on the later flight.

As I say, this is a normal accommodation for most airlines, especially for their elites. But there have been varying reports on this forum about DL agents' ability to do it, or do it reliably without you losing your seat on the original flight. I'm guessing that Deltamatic does not let them do it very easily, hence the reports of screw-ups and refusals.