To edit converted leads, you need to have “View and Edit Converted Leads” permission enabled at the Profile level or assigned through a Permission Set.
When an Account associated with a converted Lead is merged, Salesforce updates the ConvertedAccountId and SystemModStamp fields on that Lead. This can cause the Lead to appear multiple times in queries ordered by SystemModStamp.
Notes:
Mass Update Lead fields on converted Leads
Let Users View and Edit Converted Leads
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