Yes but DL I believe that taking back VS's DTW - LHR and one of the daily ATL - LHR services so its just a transfer of routes with little difference in flying hours. (I believe that there is an agreement between DL & ALPA limiting VS flying but others will know more.)
Right VS has premium economy product. I think that DL is introducing one but right now only has extra legroom seats. I don't know if this has anything to so with flying the SEA route where the main competition is BA.
The swap would appear to be logical - DTW & ATL are major DL bases and many passengers will be transferring from domestic DL flights so will receive the same product throughout. DL doesn't have the same brand awareness as Virgin in the North West so it makes sense for VS to fly it. Just my thoughts. Everything may change again in a years time.