They would be best with fresh compost next year but if you like you could leave the pots to dry out and store them like that - but repot in the spring. They will take up less space if you lift and dry, covering them with dry compost , sand etc. Beware mice.

I'm afraid I'm very lazy when it comes to dahlias, just leaving them in the pot and placing in a shed to dry out. Must confess that I don't even change the compost in spring although they get fed every weekend. The displays seems to get bigger and more floriferous by the year.