If you started Mail and it tells you that your plugin got disabled you have to move the plugin first from the disblaed folder back to the “normal” folder:
mv ~/Library/Mail/Bundles\ \(Disabled\)/TruePreview.mailbundle ~/Library/Mail/Bundles/
after this you need to figure out the new UUIDs with these two commands:
defaults read /Applications/Mail.app/Contents/Info.plist PluginCompatibilityUUID defaults read /System/Library/Frameworks/Message.framework/Resources/Info.plist PluginCompatibilityUUID
These two UUIDs need to go into this file:
You can either do it with
defaults write ~/Library/Mail/Bundles/TruePreview.mailbundle/Contents/Info SupportedPluginCompatibilityUUIDs -array-add $UUID
or any other text editor.
If the bundle is installed systemwide of course you have to look into /Library instead of ~/Library.