Previously, users with both the "Set Audit Fields upon Record Creation" and "Update Records with Inactive Owners" permission were able to view and update converted leads. Those permissions no longer grant access to converted leads. To let marketing and sales operations users access converted leads, assign them the "View and Edit Converted Leads" permission instead.
The locations where converted leads appear depends on the interface you are using.
|Where can I see converted leads?||Lightning Experience||Salesforce Classic||Salesforce1|
|Recent Items list|
|Recent Records list in the Global Navigation tab|
|Global Search Results|
|Global Search Most Recently Used list|
|Recently Viewed list views|
|Campaign Member related list (only leads added after conversion)|
|Custom Lead lookup fields|
|Recent Records on Home page|
You can’t unconvert leads.