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If you changes the owner of a filter or dashboard with shared ownership, and the new owner is not among the old co-owners, the old list of co-owners is not erased immediately. Instead, its functionality is disabled, but it is remembered until the new owner makes any change on the filter/dashboard sharing page. If the ownership is transferred back to one of the old co-owners without doing such changes, the old co-owners list will be re-instated.

This means that, as an administrator, you can transfer a dashboard or filter to yourself, make changes to the dashboard’s gadgets or the filter’s JQL (without editing the shares at all), and then give the dashboard or filter back, without erasing the old co-owners.