The docked capsules are rated for about six months, due to chemicals aboard that can degrade or escape. So if a replace vehicle doesnt arrive in time, then the astronauts may be forced to return. I recall a complete evacuation of the ISS almost happened after the Columbia accident because there werent enough Soyuz in the pipeline on Earth. The 20-some modules of the ISS are finicky enough that you dont want to leave the ISS empty for too long or it may catastrophically fail.
A lesser concern is that seats have mould inserts specific to the shape of each cosmonaut. I am not sure how they handle swaps. Maybe store the moulds on the ISS.
The seat liners can be installed in Soyuz on-orbit. When the Shuttle was delivering someone who would return on a Soyuz, they'd bring the liner up as cargo on the Shuttle and swap it in. I imagine they routinely do the same for crew who arrive and depart on different Sayuz (which happens fairly regularly.)
A lesser concern is that seats have mould inserts specific to the shape of each cosmonaut. I am not sure how they handle swaps. Maybe store the moulds on the ISS.