Delta will email you if you have it set up in your profile. I have a bunch of "Your Delta Itinerary has changed" emails archived as proof. However, if your flight is more that 2 months out, they won't email you until the 2 month mark.
I usually book 10 months out on Delta so that's probably why I am not alerted but it makes no sense not to tell people ASAP so they can get the best flights and seats, that is one reason why I book early to begin with.
I have also had plane type changes where Delta broke up passengers in the same itinerary and we were unable to sit together but they still had premium ( not 1st class, forget what Delta calls them) seating open.
If I was notified of the change as soon as it occurred it's possible I could have found seats together.
Now I log into Delta once a week to check things myself