We find out about someone leaving a company either directly (e.g. auto-response email that may or may not indicate who has replace the departed lead or phone call by our rep) or indirectly (e.g. alert email indicating that a name duplicate was created).
In either case, the we do not delete the existing lead record nor do we change the information to that of the lead's new company; otherwise, we would lose or confuse our sales and marketing performance metrics.
Instead, we have our reps change the Status field value to "Gone", which triggers a smart campaign to change the lead's Revenue Cycle Model Stage value to "Bogus Info", so we can exclude that record from future marketing. If a new lead record hasn't been created via form fill, import, etc., we have our reps create a new record in SFDC for the lead.