Overall the visual adjustments are not a massive overhaul, and some people may fail to realize what the differences are between the new look and the current design that some people will still be seeing.
The Play Store itself is also not getting a redesign. This is just for the account switcher portion itself. Since this is also just now rolling out it may still take days before its fully deployed to all users.
The account switcher in the Play Store moves to the main page
Prior to this material theme design for the account switcher it would be sitting in the slide out menu that you access from the left. Alongside all of the icons for things like Play Pass, Notifications, My Apps & Games and more.
Now it sits on the main page of the store and is always going to be sort of in view. Instead of being a drop down from the slide out menu, you'll now tap on your account icon in the top right corner of the app. The icon will be at the right end of the search bar.
Tapping on the icon will then open a card style box overlay on top of the page you're on. You can see all of your existing accounts that are available to flip between, and there are buttons for managing accounts and adding accounts as well.
That last bit is actually new. The old style design without the material theme didn't allow you to add accounts or manage accounts. So Google has essentially made these two things easier to access as well because you used to have to access them in a different location.
The voice search icon is now hidden initially
Where your profile icon now sits is where the voice search icon used to be. It's still there, but it's initially hidden from view and will only show up after you tap on the search bar.
Once you do so you'll be taken to the option to type in your search, but if you back out to the main page the voice search mic icon now shows up to the left of your profile picture.
Eventually users can also likely expect everything from the slide out menu to move into the new card overlay alongside the account switcher. For now though you can still access all of those features from the same place.