Well I'm not an expert but the "Clockwork Recovery Mod" wiping process of the ROM (data, apps, battery stats, dalvik cache, etc) takes care of uninstalling your ROM.
Then you would "install" the new rom by flashing it using "ROM Manager".
P.S. After setting everything up, you could then use a file explorer app like found on fresh MIUI installs to delete any old folders/files of apps, music, video, pictures, etc found on your SD Card. Be careful and only delete what you know you installed on there and is leftover on your SD Card.