It is recommended to un-install the old version through Add/Remove Programs then install the new version.
Why? The auto-updater run and updated my MBAM version without problem and without uninstalling the previous version...
The, check for updates process starts of downloading the full signature and program update, at the same time it displays a news item, about version 1.9 the updater does mention A Reinstall is strongly advised. Unfortunately there is nothing to cancel the download in progress.
However, it does display a shut down MBAM to complete the program update, on acceptance it closes MBAM and starts another download, but this time it is only the signature update. The about tab shows version 1.29 update apparently installed/updated successfully.
So despite the recommendation to reinstall, the update went fine, however there are no horror stories or why it is best to do the reinstall in the link provided. If it is so critical to reinstall, then the update shouldn't download the program element, just the signatures or nothing at all. So some work needs to be done on the update process.