Once a Lead is converted it can not be modified via the UI, DataLoader or APEX. However, SFDC Help has a workaround that is documented here.
Here is the doc:
Although converted Leads cannot be updated by design, you can export, modify converted leads externally in Excel, delete the existing leads and then re-import them with changes back into Salesforce as new records.
Note: It is a recommended best practice to ensure that you have a up to date or recent full data export file available before starting this process. See Exporting Backup Data for more details.
Create a Case with support to have the "Create Audit Fields" feature activation enabled for your organization as this will allow you to set the audit fields Created Date and Last Modified Date upon record creation:
Include the below information in the feature activation request:
a. Organization ID
b. Requested feature
c. Requested duration
d. Business case
Perform an export on your converted Leads with Data Loader. See Exporting Data for more details.
- Update the necessary values on the CSV export file from step 2
- Delete the converted Leads that you exported in step 2
- Import the converted Lead information with the updated .csv file from step 3
Essentially the above creates "new" Leads with the updated information while keeping all of the record associations and date/times of the original Leads.
Note: The Lead ConvertedDate field is a date field while CreatedDate field is a date/time field. You may receive an Error: FIELD_INTEGRITY_EXCEPTION: Converted Date: Converted Date(Sat Sep 06 00:00:00 GMT 2014) Before Create Date(Sat Sep 06 17:17:44 GMT 2014) for Leads that were created and converted on the same day. To correct this error set the affected record's CreatedDate value back one day or their ConvertedDate forward a day. Even if you attempt to change the time attribute of the ConvertedDate field to after that of the CreatedDate field you will still encounter the error because despite whether you may specify a time attribute for a date field, it will always be set to a default value of 00:00:00 GMT or 00:00:000Z on import.
SFDC Help actually has quite a few useful Knowledge Articles dealing with common and not-so common situations.