Is there a way to assign a completed task to a project?
A completed task would have already been assigned to a project, if that was only the Inbox or not. Once they are completed, the only way to change that would be to un-complete them, then change the project.
Thanks for the very fast response - how do you un-complete a task?
Head to the project it was part of, at the bottom there is a circle clock icon, this brings up completed tasks, those will be uncheckable.