Migrating already-converted Leads into Salesforce involves writing to the fields listed below.  These fields are normally read-only; submit a Case with salesforce.com Support to activate the "Create Audit Fields" permission, which enables these fields.

These fields are writable only when inserting new records, not when updating existing records.

ConvertedAccountId and ConvertedDate are mandatory.

Salesforce.com allows these fields to remain editable on a Production org only for the length of time needed for a single migration.

Salesforce.com allows these fields to remain editable on Sandbox orgs for longer intervals.