If you have the storage available, use DriveImageXML
to make a backup of your entire drive, just in case there are configuration files/directories that you overlooked during your backup.
If you're doing an in-place upgrade, Windows won't purposely remove your files or bootloader. (Unintentional errors like power failures can still occur.) So backup whatever you're comfortable with.
If you're planning on wiping the entire partition, disconnect any other physical drives that aren't being used, as Windows can sometimes install the bootloader to a different drive. This should keep the files on your other drives safe.