1. It is possible to manually share a record to a user or a group using Apex or the SOAP API. If the owner of the record changes, the sharing is automatically deleted. Salesforce currently removes all manual sharing from a record when the owner changes. If you are manipulating sharing via apex code or simply sharing to portal users, you might want the sharing to remain when the record owner is changed.
2. Changing your sharing model deletes any manual shares your users have created.
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