Whenever they say "available today" I take it as "hopefully I'll start seeing it in the app UI by tomorrow" rather than "I should get my hopes up it's there now".

When they do push the update to the app UI to me I expect 5.2 Instant will be moved under the legacy models submenu where 5.1 Instant is currently and the selection of Instant in the menu will end up showing as 5.3 Instant on close (and it'll be the default instant at that point).