Thanks for your advice, Frederic. I guess I just dread the boring first year of the MPhil (with only Micro, Macro, Econometrics and Math). I understand that that's what is considered a "rigorous" training - thanks to the obsession with math at US universities - but I don't want to waste the next two years of my life dealing with form only, not with content. The Graduate Institute at Geneva has an interesting program where in addition to a rigorous training in the core disciplines you can also take classes in areas such as migrational economics, resource economics, etc. and it's even possible to take a handful of courses in other areas (politics, international affairs, history, development studies). For someone who is not sure about doing a PhD and staying within the bounds of economics this seems to open more opportunities...
I just called both the Economics and the Development Department at Oxford and they say it is technically possible to change from the MPhil to the MSc but depends on whether they still have spaces available in May or at the beginning of the term. They even said it has been done in the past (last year there was a case exactly like mine) so it's definitely in the realm of possibilities. Would love to hear if anyone has ever done this or knows of someone who has switched.