not entirely true. Jet may be not so frequent with it or open about it, but push them to the wall and they will do it. I've had an offer a couple of times from them. Last being a year and a half back when at Delhi they offered to endorse me over to SG due to a massive delay on one of their flights to BOM which i was booked on.
Oh, and I think I was silver/gold then. not Plat
Not sure if it still holds true.. Earlier, I frequently used to change and take earlier flight, on reaching airport early enough, but now I am refused, even if I am a plat..
In any case, I don't think it really matters which tier one holds these days. Jet is agnostic to membership status, and customer service is uniformly pathetic.