if your tkt when booking is 3ac then after chart preparaton it shows 2ac then its upgraded class tkt no need to pay extra fare to tte.
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if your tkt when booking is 2ac then after chart preaparaton it shows 3ac then its downgraded class tkt , u can ask tte for compensation of fare (excluding 3ac fare / some amount u may recieve if u wish to travel in 3ac or u can cancel your tkt if u wish full refund)