Can I edit a job plan that has been signed off?

In most organisations, you can only edit a job plan that has not been signed off in any way. 

If, for any reason, you need to edit the job plan any further, you should contact your Job Planning Administrator, who will be able to adjust the status of the job plan to allow further editing.

NOTE: If the job plan only has your signature against it, you may be able to remove the tick against your own approval but if there has been sign off from anyone else in the approval chain then this will not be possible.