I'm doing some restructuring of our data and field values. I'm currently cleaning up our lead source values but am having some issues updating values on converted leads. Has anyone figured out how to update lead source values on converted leads, I haven't been able to find any solutions on the SFDC forums.
So what I'm trying to do is consolidate some of our lead source buckets and add lead source details. I have a lead that has an lead source of "Event - Event Name - Year", the lead was converted to a contact and the lead source value was written in from Marketo (it was not a data value in SFDC). What I am changing is the lead source to "Event/Conference" with a lead source detail of "Event Name - Year". I'm able to make those updates on leads and contacts via a data management campaign in marketo. The issue is around a lead that had the original source but was them converted to a contact. I can change the contact lead source but if I'm running lead reports the original source will still appear on leads that were converted.
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.