I am not a dahlia grower but for the hydrangeas, I would not worry much. They should be fine if you always maintain the soil well mulched (2-4" of organic mulch) and if you water deeply the night before dips into much colder temps. Do note that some early frosts or dips below freezing could zap the foliage. Covering the plants and watering can help a little. If temperatures go much more below freezing in the future, hydrangeas that bloom on old wood may need winter protection to bloom reliably in Spring.

How big are your dahlias? Ground temp is not a worry just air temp below freezing.

tj

The top growth of the dahlias will go to mush if the temp drops to freezing or below. Cover them and hope for the best. I can’t see them recovering from being dug up in full growth and then replanted.

Please leave the hydrangea alone so it can continue to develop its root system and make through winter and be raring' to go next spring. These shrubs develop a substantial proportion of their root systems in fall. Someone usually pitches here about what proportion of root development happens in Fall.