There are people bold enough here to ask for anything. The LHR-ATL-BOS request greatly exceeds the Maximum Permitted Mileage of a half a dozen DL coach fares I checked but they may cut you some slack.

You are entitled to:

- a refund to original form of payment because of the change in carrier (the sub of Virgin Atlantic for Delta)

- a refund to original form of payment because of the significant change in time of departure (or arrival)

You may not find the refund and rebooking on DL or another carrier prudent, depending on available fares.