Changing tax regime within the same year

An employer has been deducting TDS on the basis of old regime for the previous three quarters. But in January, calculation made by the accountant shows that the new regime is beneficial for most of the employees, even after the consideration of the deductions allowed under the old regime. Are employers permitted to change the basis of tax deduction within the year, from old to new, or is it like the employer has to follow only one regime for the whole year (and the employee can get refund if he wants, after filing taxes)?