Sorry for such a late reply. Perhaps you have already figured it out. In case,
If you currently have Linux Mint 8 Helena main, and Linux Mint 7 XFCE CE Gloria, and you want to remove both and install Linux Mint 8 XFCE CE only:
Just fire up the LiveCD for Linux Mint 8 XFCE CE and you can choose to use the whole disk for installation.
If you wanted to upgrade your current "Linux Mint 7 XFCE CE Gloria" to the upcoming new version - I am unsure if an upgraded is recommended, or possible without many problems, you may have to install with the LiveCD.
Keep an eye on the Linux Mint main website and blog for the release notes and any possible upgrade instructions.