PhpStorm fails to start after in-place upgrade: safe rollback without losing settings

Situation

After an in-place PhpStorm upgrade, the IDE no longer starts reliably, and I need to return to the previous build. My understanding is that JetBrains stores configuration, plugins, caches, and logs in version-specific directories (e.g., ~/.config/JetBrains/PhpStorm<version> on Linux, with equivalent paths on Windows and macOS), so the prior version's settings should still exist on disk.

Constraints

I want to avoid deleting the entire JetBrains directory, since the config folder holds settings I need to keep. I also edited custom VM options via Help > Edit Custom VM Options at some point, and I understand a stale .vmoptions file is a common reason an upgraded IDE fails to launch. The Toolbox App appears to retain older builds for side-by-side installation, which seems like the intended recovery path.

Open decision

The part I cannot resolve is plugin state: several third-party plugins were updated for the newer build, and there seems to be no automatic downgrade of plugin versions when rolling back. I am unsure whether the downgraded IDE will refuse to load them, silently disable them, or misbehave.

Questions

• When downgrading via Toolbox, is removing the newer version's config directory sufficient, or must plugin folders be cleaned manually?
• Should I delete the custom .vmoptions file before or after reinstalling the previous build?
• Is exporting settings (File > Manage IDE Settings > Export) still worthwhile if the IDE only starts intermittently?