It's written wrong. One-class ugrade means upgrade to next higher class. Eco to PE, PE to business etc.minhazuddin
I am not sure about this but sorry if i am missing something
as IDFC mentioned that the upgrade voucher they will give apart from silver membership for that upgrade voucher you can use it only on premium economy or above class is that so
below image u can check not sure they have written just for example or explicitly they need specific class ticket to use that voucher
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It is deliberately written like that as a part of marketing strategy. If it was written eco to PE instead, it would have looked less appealing to potential customers.