It would be unusual for them to allow a change from BOS to ATL as the starting AND end point for the entire trip. This is because ATL is not an obvious co-terminal to BOS. Maybe if you asked to change to PVD-ATL-LHR and return, or MHT-ATL-LHR and return, that would make more sense, because those airports are within 60 miles to BOS (and the closest DL serves to BOS).

Either that, or you could do as Widgets suggested and ask for LHR-ATL-BOS (ideally the LHR-ATL leaves around the same time as your original LHR-BOS, so you can make the argument you need the same departure time) and then take the segments as you wish (as you have to pickup checked luggage at your first US port of entry anyways).

It will all depend on the agent you speak to, how busy they are at the time you call (which means how closely they pay attention to the your rerouting versus just wanting to get your reaccommodated quickly and on to the next caller), and how nice you are. Because the fare rules will allow you a refund at any time or a reroute specifically to BOS on other flights of your liking. Anything beyond that is a favor on DL's behalf, hence the subjective factors I've just listed. But if you make a reasonable case, more often than not DL is willing to work with you as long as you do not act like a DYKWIA.