The answer to that might be in a confusing part of the email I got. Below the section where it said "Your Premier Membership has been upgraded to a Platinum Membership" and listed the new benefits (11 parks, four guest tickets, and 20% off dining) there was another section with contradictory info:
View attachment 20485
Those aren't normal Platinum benefits, plus the section right above said I now get 20% off shopping and dining, not 30%...so maybe these are what current Platinum members are "upgraded" to? Not sure why it was in the email I received as I wasn't a Platinum member before.
I got two different images in the email saying I got 20% off and right below 30%. I'm on the grandfathered premier membership plan from a few years ago when they first rolled that out as the highest tier before they changed it the next year to platinum membership being the highest.
Here's how mine looked...