I'm looking to fly ARN-LHR-PEK in a week or so to kick off my DONE4. Expertflyer shows BA flt. 777 ARN-LHR as an A319 at 11:55a with D0, but AY flt. 5927 ARN-LHR as an A321 also at 11:55a operated by BA with D4. These are presumably the same flight (as an AY codeshare) -- not sure why the difference in aircraft. Assuming they are the same flight, can I book this flight as AY 5927 even though I'm continuing on with BA from LHR-PEK?