It doesn't matter if you're going to do a clean install or not, back up you data anyway. You don't have to format anything just because you get a window with a message to do so. Don't let software rule you.
Click Time Machine icon in the menu holding down Option Key. You'll get see a "Browse Other Time Machine Disks" option, which will allow you to access your old backups. You'll have to make a new backup for a new install, however.
Oh ok, thanks. I have a lot of data on my external passport HD and when I saw "format" I was like..oh heck, no.