Create a 'Undo customization' button for managed solution components.
Once a managed component from a third party solution is customized, this (unmanaged) customization will always be effective, even if the managed solution is updated.
Though this is intended behavior, there is no way to undo these customizations. This means that any update of a managed solution will not be effective. The only way to get the solution update effective, is to import the solution with 'overwrite customizations' applied. This method has to major disadvantages: 1. this applies to the whole solution, which is in most cases not desired, 2. this is a one-time solution, as this will be needed for any future solution update.