By design, Lead items are deleted once they have been converted to Account items. This behavior is controlled by the following Advanced Flex Setting:
Setting Name: DeleteLeadAfterConvert
Setting Value: True / False
The default value for this Setting is “True” which means that the Lead item will be sent to the recycle bin for deletion once the conversion to an Account has taken place.
Should your preference be to have the Lead item remain in the Leads list, use a Setting Value of “False”.
If the preference is to retain Lead items once converted and not delete them, add a Column Field (Yes/No) to the Leads List, titled “Converted” (see image below):
Use a default value of “No” so that only those Leads that have been converted will reflect a “Yes” value:
The following screenshot demonstrates a Leads List in which the items are not deleted upon conversion, and the “Converted” Column field has been applied.
In this scenario, new Lead items will contain a default data value of “No” in the Converted column. If the Lead is converted to an Account, this value will automatically change to “Yes” so that you can differentiate Leads that have been converted. (Be sure to add this newly created Column to any necessary SharePoint Views of the Leads List.)
Note: If a lead item is left in the Leads list when it is converted there will be no ability for the user to once again convert it since the Convert Lead button is removed.