Honestly, the 2025 changes are actually good news for most salaried people, especially those in the 15-20 LPA range like you. Let me break it down simply.
The new regime is now more attractive. Standard deduction has been bumped up to ₹75,000 from ₹50,000. The slab structure also got revised — zero tax up to ₹4 lakh, then 5% from 4-8L, 10% from 8-12L, 15% from 12-16L, and so on. The big one: if your taxable income is up to ₹12 lakh, the 87A rebate now covers your full tax liability. So effectively zero tax up to 12L in new regime. For many people this alone is a game changer.
Old regime is NOT gone. They kept it. You can still choose it if you have big deductions — 80C investments, home loan interest, HRA, NPS contributions through employer. But here's the thing most people get wrong: they assume old regime is always better if they have deductions. At 18 LPA CTC, unless you have a home loan of significant size plus maxed 80C plus HRA, new regime will likely win now. The improved slabs and the 75k standard deduction tip the balance.
For your specific situation — 18 LPA CTC, Pune — I'd say quickly add up your actual deductions: 80C (max 1.5L), HRA if applicable, home loan principal and interest if any, 80D for health insurance. If your total deductions are under 3.5-4 lakh, just go with new regime. You'll probably save more.
About the company form — just declare new regime for now. You can still switch at the time of actual ITR filing in July if you want to recalculate. The declaration to employer is mainly for TDS purposes, it's not permanently locking you in.
One thing I'll say from experience — don't let your CA just default you to whatever. Ask him to show you the actual number comparison on paper. Some CAs are still pushing old regime out of habit. Run the numbers yourself on the income tax department's calculator on incometax.gov.in, it's actually decent now.
My recommendation: go new regime, stop stressing about it. At your income level with no major home loan, you'll almost certainly pay less tax this year.