I wanted to disable the prompt and button for new Outlook. I found this guide and wrote a few lines of PowerShell to automate it and wanted to share.

# Source https://edi.wang/post/2025/1/20/how-to-stop-automatically-switching-to-new-outlook
$path='HKCU:\Software\Microsoft\Office\16.0\Outlook'

# Remove and create to ensure correct data type
Remove-ItemProperty -Path "$path\Options\General" -Name 'NewOutlookAutoMigrationStage' -ErrorAction SilentlyContinue
New-ItemProperty -Path "$path\Options\General" -Name 'NewOutlookAutoMigrationStage' -Type DWord -Value 1
Remove-ItemProperty -Path "$path\Options\General" -Name 'NewOutlookAutoMigrationStage'
Remove-ItemProperty -Path "$path\Options\General" -Name 'NewOutlookAutoMigrationType'
Remove-Item -Path "$path\NewOutlook"
# Remove and create to ensure correct data type
Remove-ItemProperty -Path "$path\Preferences" -Name 'UseNewOutlook' -ErrorAction SilentlyContinue
New-ItemProperty -Path "$path\Preferences" -Name 'UseNewOutlook' -Type DWord -Value 0
Remove-ItemProperty -Path "$path\Preferences" -Name 'NewOutlookRenudgeStartDate'
Remove-ItemProperty -Path "$path\Preferences" -Name 'NewOutlookRenudgeStartDate'