I'd have to say that my preference is for just the US to have been swapped out... mostly because part of the storyline can be some of the Fen trying to help pick up the pieces elsewhere on the planet. Things can be a lot more chaotic on the ground, unlike a whole planet swap where more countries can actually band together against the Fen who suddenly appeared in space and raise the spectre of 'damned alien invasions'.
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"You know how parents tell you everything's going to fine, but you know they're lying to make you feel better? Everything's going to be fine." - The Doctor
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"You know how parents tell you everything's going to fine, but you know they're lying to make you feel better? Everything's going to be fine." - The Doctor