On my side, the upgrade is not smooth at all, the IDE ignores the modifiers most of the time until I edit the code and revert changes.
I reported the issue here 2 days ago (with screenshots and details), but it hasn't been assigned yet:
https://youtrack.jetbrains.com/issue/KT-42274
Can it be looked into? I'm wondering if it's caused by changes that are designed to improve performance, which would do it at the expense of correctness… and UX in the end.