NRI status immediately after commpleting 182 days or FY wise ?

As per Income tax act you remain NRI for complete financial year, if you remain less then 182 days in India, in that year. (that being one of the conditions as per the act)

Check that you remain maximum 181 days in India including days of arrival & departure, to qualify for NRI status IN THAT FINANCIAL YEAR. 

Thanks than in my case by what date I need to change status of my saving bank account / Demat / Trading account to NRI ?

If you have stayed less then 182 in India in FY 2024-25, you will be NRI for  FY 2024-25. For FY 2025-26 & onwards, we need to check your stay in India for that particular year along with your total stay outside India for earlier years. Most probably would become RNOR for that year if you are returning to India after many years of stay outside the country.

Refer: https://cleartax.in/s/income-tax-act-residential-status