Summary:
Timesheets remain the same because existing timesheets are not affected by changes to the system timesheet period.
When you modify a timesheet period settings, only timesheets created after the system was modified will be affected by those changes.

Resolution:
To change the settings for an existing timesheet, you must delete and then recreate that timesheet.

Note:
Before deleting any user timesheet, please ensure that all the time entries are noted down or exported through reports.