You wont lose any saved files unless you choose the option to reformat your disk.
When you first insert the disk it will gather data including compatibility of existing file system. You get adequate warning and can go 'back' at any time.
You also get options to save all your existing program preferences/cookies/temp files/defaults etc. etc. Probably best to do so. Last time I 'reinstalled' (which amounts to the same procedure) I hit the wrong button in my hurry at that point and then spent a day retrieving all my old settings manually, something I would not advise anyone to do if they are not computer literate.