Type of aircraft is not guaranteed. Carriers change aircraft routinely, adding or subtracting capacity, to balance maintenance and refurbishment needs, to balance crew utlization...

If the change to arrival or departure time exceeds 90 minutes you can get a refund. In that situation DL would likely allow you to reschedule on any DL-operated flight on your ticketed travel dates.

If DL changes carrier (as with substituting DL Connection for a DL mainline flight) you can get a refund.

If you buy a ticket directly from DL, DL is responsible for notifying you of schedule changes. That may not take the form of an email until ~30 days before flight.