Wiggy - yep, the couches (the moulded parts, at least) are switched; I think Michael Foale discusses it in Dragonfly.
When he flew, the Soyuz had an on-orbit lifetime of something like 6 months; for the longer-duration missions, they'd fly up a crew in a new Soyuz, who would then return in the older one (which is also how the various ISS "space tourists" got their flights). I think I read that the on-orbit lifetime has now been extended to 12 months, but could be wrong about that...