As murraymints says, lift the tubers but I would say when replanting go easy on the fertiliser because too much nitrogen can cause over-production of lush green leaves, with few if any flowers. Best to use a fertiliser that is high in phosporous, and low in nitrogen. Most tomato fertilisers would have this balance, so you might try one of those.
Also, dry conditions can prevent dahlias from flowering, so when you replant make sure you keep them well watered.
Good luck with them. If you get it right you will be well rewarded. Dahlias are beautiful flowers, and I love them!!