我看条款里写着只要机票是开卡90天内的行程，可以不需要用这个卡支付也可以有行李额。所以从UA Gateway升级到Explorer的90天内也符合吗？
To receive first standard checked bag free, the primary Cardmember must use their United℠ Explorer Card to purchase their ticket(s) (this purchase requirement is waived for flights completed during the 90 days following the account open date) and include their MileagePlus® number in their reservation.