Don't forget to archive your parts.txt file as srs5694 explained, although so long as the Ubuntuforums server doesn't fail us you now have a backup in this thread!
Yes, backing up data is a very good idea. As srs5694 said, running fixparts or sfdisk is unlikely to affect data within partitions, but it's always a good idea to make backups whenever you do any partition editing or changes. And if you haven't archived your data yet, now's the time to do it!
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