The Effective date (Start date for first policy) of the timeoff policy would remain as the original start date of the user, even if the start date of the user is changed in the profile.
The effective date has to be changed manually to the new start date, if the change is being made a long time after the policy has been set up. This to avoid making changes to the existing balance.
However if the policy has been set up recently and making changes does not affect the timeoff balance, then the Reset to Defaults can be used to reset the settings. This would update the effective date to show the new start date of the user.