It might technically be an "upgrade", however if the OP's account is true, then the cashier lied multiple times during the interaction. First was when the OP specifically asked whether this was an app for a new card and the cashier confirmed it wasn't. Then the cashier promised a status upgrade to platinum and the card ended up being a downgrade to silver.
It is basically a case of he-said she-said since there's no audio recording, but regardless there's a strong whiff of fraudulent conduct here. It's one thing to lie by omission and then later on point to some fine print tucked away and say that the consumer should have read this or that. But it's quite something else to straight up say something false to get the consumer to undertake a transaction.
Problem is, the customer didn't get to see the fine print because the salesperson took away the pad, which is illegal. They don't get to point at it later.
OP: if you don't want the card, I would contact corporate immediately while driving down to the store. Most stores only keep recordings for 2 weeks max. Have your receipt for that purchase to show the date, time, register, and employee. Corporate can get to someone at the store quicker than you. Driving down there is just a back up.