With Drupal’s new versioning and release planning came the promise of easy upgrades between major versions. No more major database overhauls. No more rewriting business logic just to keep things working. No more major investments in expensive migrations to maintain feature parity.

Upgrading to Drupal 9 from Drupal 8 provides the first test of these promises. Have the promises been fulfilled? 

Yes.

Is it as simple as flipping a switch?

No.