Anyway to automatically re-nest moved subtasks?

I have a project where tasks and subtasks begin in a "parking lot" section. Then when their custom fields are changed, they move to the relevant section using rules. When they are marked complete, they are moved back to the parking lot and their custom fields are cleared. But when this happens to subtasks, they are un-nested and duplicated. Is their anyway to use rules to re-nest or delete the duplicates?