if there is a seat available then you can upgrade to j but keep in mind that it will not be at a discounted price....you will have to pay the regular difference between your fare & the best available business fare....

you can also use vouchers or miles to upgrade at the airport....

your free baggage allowance in economy is 20kgs & for business its 30kgs....for domestic flights, excess baggage charges per kg. will be calculated at the rate of 1.5% of the economy class normal fare....

so basically if your are 17kgs overweight & lets suppose your ticket cost inr 8k then the excess baggage would be inr 120 per kg so inr 2,040 in total....if you want to upgrade to business then the fare difference will be anywhere between inr 10k-15k....