If you do an Upgrade or Archive and re-install type upgrade you should be safe, that being said, like Foamcow said, it is a really good idea to backup your macbook before you update.
Carbon copy Cloner or SuperDuper are to good methods, providing you have an external drive you can backup to. If not you could burn disks of your docs and photos.
Twist
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Everything will be OK in the end.
If it's not OK, then it's not the end