Maybe I'm wrong but it seems that UA has a shortage of international equipment at the moment - having to put a temporary 747 on the schedule last summer, dropping routes such as LHR-DEN which run consistently full because the equipment can make more money elsewhere etc. etc. If that is the case, then I cannot see that they would replace a 747 with two 767s, unless there's an equal and opposite swap elsewhere. As many of the 767s operate thin routes (eg IAD to smaller European cities), they won't want to put the 747 on that route.
I suppose that switching some 767s to 757s might do the trick, thus pulling some domestic capacity to international, but otherwise it's not going to happen.