Use mass ownership change when you need to transfer one user's data to another user. This can prove useful when, for example, a colleague is leaving and you need to transfer his records to someone else. The change takes a few clicks and can only be done by an Administrator.
Click on the Settings button in the top right corner under your name.
Select Mass ownership change from the menu on the left.
Select the original owner and the new owner, and mark the items you wish to make the mass change to. If you are using security levels, set the correct level as well. Finally, click the Change Owner button.
The change will be reflected in all selected records, except activities that have already been completed or canceled. These will remain under the ownership of the original user, so you will not lose information about who actually carried out the activity.