The start-of-year declaration
Employers collect your regime choice (and proposed investments, for the old regime) early in the year, so the monthly TDS matches your likely tax. Declaring accurately avoids large over- or under-deduction across the year.
Default and the right to switch
If you make no declaration, the employer applies the new regime by default. Importantly, the choice you give your employer is only for TDS convenience — you can still pick the other regime when you file your return, as long as you have no business income (business owners face restrictions on switching). Confirm the switching rules for your situation.
A worked example
Example: you tell your employer ‘new regime’ for simplicity, so TDS is deducted accordingly. At year-end you realise your home loan and 80C make the old regime better — you simply choose it in your return and claim the larger refund. The employer declaration didn’t lock you in. Our team can confirm which regime to file under.