...or will apps built with ARKit 3.0, or even any version of ARKit that has plane detection, automatically get the upgraded functionality? The overview is vague, says "Instant AR placement is automatically enabled on iPad Pro for all apps built with ARKit, without any code changes."
Can you cite the apple.com source where this is stated?
https://developer.apple.com/augmented-reality/arkit/
Instant AR
The LiDAR Scanner on iPad Pro enables incredibly quick plane detection, allowing for the instant placement of AR objects in the real world without scanning. Instant AR placement is automatically enabled on iPad Pro for all apps built with ARKit, without any code changes.
> The overview is vague
Not sure I see that...
That link makes it clear that the topic is ARKit 3.5
Cool, thanks. Because ARKit uses the LiDAR scanner to improve the speed of the original (not new) plane detection, you just need to rebuild your app with Xcode 11.4 (with the ARKit 3.5 SDK), and with no code changes, ARKit apps will detect planes much faster on the new iPad Pro.