I think the 8th AF could have gone to the night role very easily, as they were already 'area bombing' cities in daylight on numerous occasions
To look first at the 8th in the night role, BC had H2S and the 8th had H2X, which meant both could bomb a city-size target in zero visibility conditions, with enough accuracy to provoke a firestorm. The 8th, although always depicted as making 'precision' attacks did in fact 'area bomb' on radar on a number of occasions during the war, which essentially guaranteed the same king of results BC got bombing at night, i.e widespread and intentional damage to civilian as well as military assets in the target area. Even the lesser payload of the B-17 needn't be a terminal handicap - as long as the right 'mix' of incendiary to large-capacity HE bombs is observed, reduced bombloads will still be capable of starting Hamburg-Dresden style firestorms, although the process may be somewhat slower. The real diffculties will be outside the 8th AF itself, in providing an adequate Pathfinder force, NF force and onboard ECM for the bombers - all things that the USAAF never had a pressing need to develop.
As for BC operating in daylight - this is a bit more difficult. I don't believe defensive armament is really that big an issue. The idea that the firepower of bomber machine guns alone could defend a formation from fighters was put to bed over Schwienfurt. Adding waist guns and ball turrets to Lancasters would not make a box of Lancs 'fighter-proof' any more than it did for B-17s, and the extra weight and expense in terms of equipment, ammo and crew actually reduces BC's offensive potential. What they really need is a long-range escort. But just as the USAAF never really needed to develop night bomber ECM, Fighter Command has never needed to escort bombers to the other side of Germany and back, so they have no suitable type. As the USAAF would not need the P-51, production could be diverted to BC, but if we are envisioning this 'swap' from very early in the war, BC is going to go through the same slaughter, and the same protracted period of operating only within fighter range until the Mustang becomes available in significant numbers...