Adding/Changing Reference to Slip After Approved Bill

Hello all,

We have recently utilized the reference section on slips to organize/track specific sets of data; now we would like to go back to apply this bookkeeping system to the slips for the entire month of August. I know that it is nearly impossible to edit a slip after the bill has been approved, but is there a way to add/change references on these old slips? Again, nothing material is being changed, just a superficial identifier that helps us run reports and sort data later on.

Thanks in advance,

Sean