IIRC the MQD $25,000 waiver is stated in terms of total spend on DL affiliated AmEx cards in FF's name (and associated with the FF account?). I know that spend across several cards can be combined for the $25,000 threshold. So IMO the question would be whether spend on a DL Gold AmEx card can count for the waiver if one also has a Plat or Reserve card and whether DL counts spend on such a card in the same calendar year but prior to obtaining an AmEx DL Plat or Reserve card.

BTW, if you've held the AmEx DL Gold card for less than a year, you can't upgrade and must apply for a new (Plat or Reserve) card. There's a consumer protection regulation that credit card issuers cannot increase the annual fee in the first year.