I was just logged in with my PRG and saw a small banner to upgrade to Plat for 50k! I extracted the link and tried with my friends' PRG and for one guy it worked but for another it didn't, so it seems to be targeted to only some PRG.
Anyway, just use this link and log in, and if it tells you "Our systems are being updated. Please try again later, or call the toll-free number on the back of your card." then you aren't eligible.
Enjoy!
https://www304.americanexpress.com/personal-card-application/upgrade/pa/platinum-charge-card/11272-WOC0000052Terms:
You Are Pre-Approved
Accept this invitation to apply and enjoy the privileges that wait for you.
Earn 50,000 Membership Rewards® points after you spend
$5,000 within the first three months of Card membership (1)
Airport Club Access Program - Enjoy complimentary access to the airline lounges of American Airlines Admirals Club® lounges, US Airways® Club, and Delta Sky Club® for you and your immediate family, or up to two traveling companions, to participating airport clubs.(2)
24 – hour access to Platinum Card Concierge service for some of the finest experiences in dining, entertainment, and luxury.(3)
Enjoy access to complimentary benefits and competitive rates with each FINE HOTELS & RESORTS booking at over 750 extraordinary properties worldwide.(4)
The annual fee for the Platinum Card® is $450. For up to 3 Additional Platinum Cards, the annual fee is $175. Each Additional Platinum Card, after the first 3 Cards, is $175. Any Additional Cards on the account will be automatically upgraded.(5)