Update Campaign after task completed

I've had to update the campaign for tasks after the task had been completed. In order to update the campaign or sub campaign on a task, you have to un-check the task to remove the completion status. This is an issue because then the completion date for the task has been updated as well when you go to hit complete again. For tracking purposes, I'd want to try and keep the completion date for something as accurate as possible.

Two potentially solutions would be to either adjust the "completion date" after unchecking and rechecking, or be able to edit the campaign after a task is completed.