Hana,
You will need to do a Replace Resource on the user within the tasks. This option is available in the Work Plan Utilities Menu of the toolbar. This will allow you to replace the user with another. If the user has any time entries on "old" tasks, they will be set as Inactive and will not be able to add anymore time because the tasks will not show up in their Timesheets and being removed as a resource on the work plan, they will not see the Tasks anymore as well.