The tax regime opted by an individual will decide how much tax will be deducted from his/her salary income. From financial year 2023-24, if you do not select any tax regime, the new tax regime will be the default option. This implies that your employer will deduct tax as per the slab rates of the new regime. So, even if you wished to avail tax benefits/deductions under the old regime which could comparatively reduce your tax liability, you will end up paying more tax. You can only change the tax regime again at the time of filing income tax returns and will then have to wait till the tax refund is processed to get the excess money back. So, if your employer has asked for investment declaration, determine which tax regime is beneficial for you depending upon your total income and the benefits you can avail, mention the tax regime and submit at the earliest. If you do not, you will be taxed as per the new regime by default.