Hey Richard,
I went down this rabbit hole a few months ago. From what I understand, it's currently not possible to edit a converted lead's data after it has been converted (so as to reflect in a leads with converted leads report). As you mention, it's possible to change the new contact's info, but I completely understand your quandry here - it's not the same thing for historical purposes.
We spent a ton of time researching it, but essentially once a lead has been converted, the data at the time of conversion is what is snapshotted for those reports. The data itself falls within the category of meta data, and is locked down from further editing (Lead source is not a unique field in this regard - the same is true for all lead fields upon conversion). We even went as far as approaching SFDC about adjusting this for us, but no luck.
My workaround has been to export and fix the data in Excel or Tableau. Awful, I know, but here's a great example of where the wheels fall off of SFDC reporting.
If you do find a solution, please keep me posted. I'd be very interested to hear of it, as it's a big pain point for us with SFDC.