Why convert a flat rate?
A flat rate charges interest on the full loan amount for the whole tenure. A reducing rate charges interest only on what you still owe. The same number costs much more as a flat rate, so offers can only be compared once they are on the same basis.
Example
₹5,00,000 for 36 months at a 12% flat rate means ₹1,80,000 of interest and an EMI of about ₹18,889. The reducing-balance rate with the same EMI is about 21.2% — that is the number to compare with other offers.
Compare offers the right way
Use the reducing-balance rate, or better, the APR in the lender's Key Fact Statement, which also includes fees. Read flat vs reducing interest rate for the full explanation.