Originally Posted by christep

Yes it is - I did exactly that with a DONE3 some years back: I took CAI-LHR with a view to then going to Asia then NA and back to Europe. However, my TA managed to snag me a seat on one of the last Concorde flights (which tells you that it was quite a few years ago, but I don't think the rules have changed in this regard, except that the Concorde supplement is no longer available), so I switched to do Europe-NA-Asia-Europe. And it was reticketed on the fly by the BA desk at LHR (after my very excellent TA had rejigged all the reservations).