Install via grub - Why clean HD?

[Scott Merritt]

Before drove everyone nuts asking yet another question about how to 
install FC2 without booting off the CD, I searched the list.

I have FC1 installed, and saw a post the mentioned copying the needed 
files from /isolinux on the first FC2 cd, and then tweaking grub.conf to 
use them since I can't boot from the CD.  I figured I'd play around with 
this strategy, except, the poster mentioned having a clean hard drive.  
Why would I need a clean HD?  Is it unable to upgrade like I'd prefer to 
do since I already have FC1 running good?

Thanks,
-smerritt