I just tried this out to see if it would work and it did
Select all or some items in a list (shift/click or cmd/click). Drag the items to Completed in the sidebar. All items get checked.
Go to Completed in the sidebar. Select items you want to uncheck. Drag the items to any list. All items get unchecked.
This could be useful for lists that you reuse like a packing list for a trip.