Make sure to backup/export your Classic Shell Menu and Explorer settings BEFORE upgrading to Classic Start.
You'll have to import the settings again after the upgrade.

To export Menu Settings:
Right-click on the Start button, and click Settings
Click Backup
Click "Save to XML File..."
Choose a destination folder and click Save (This creates Menu Settings.xml)
Click OK

To export Explorer Settings:
Click on the Classic Shell icon (far right)
Click Backup
Click "Save to XML File..."
Choose a destination folder and click Save (This creates Explorer Settings.xml)
Click OK