You have two options:

1) Export all Timesheets, update them outside of Odoo, then re-import then.

2) Create a Server Action with a line of code to do it on demand:

record.unit_amount * record.employee_id.hourly_cost or 0

Click Create Contextual Action to make it available from the Action Menu

Add the relevant Allowed Groups to secure this feature.

You would then select the Timesheets you need to update and the new cost would be applied based on the then current rate.