Not saying your fault, but for future reference if you had called to make the schedule change the agent should have been able to carry your upgrade forward to the new flights. Especially if the reason for the change was due to Delta adjusting the flight schedule, causing a too tight connection.

This has happened to me many times, and I have always been able to keep my upgrades. Occasionally, an agent says there isn’t the right ticket class available, and I politely say “thank you, let me look at my options then and I’ll call back”. And then I hang up and call again and speak to another agent, who is able to process it.

As countless others have said on this sub, it always pays to call - avoid the chat and avoiding making the change on the website yourself, unless it’s completely straightforward.