[pommes]

Two of my dahlias need to be moved before  next season. One is now in it's fourth year, & the other was new this year. When is the best time to move them; in the autumn or early next spring?

[Yorkie]

Wait until the foliage dies down in late autumn.

If it's a tuberous variety the tuber should come up over winter anyway.  If it's not a tuberous variety, then move when it's dormant over winter.