They are doing a rolling release through the end of June so it could take extra time for some tenants to get upgraded to Grail (Gen3/Strato/New UI, whatever you want to call it).
When it hits, you will see a banner on the top left of your existing UI, asking if you want to switch. This is a per user toggle and can be undone from the new UI by clicking on your Account name on the bottom left and un-toggling the switch.
I won't say any more because it's a small but fun experience when you switch.
After that though, please feel free to contribute to this post on early feedback