The only way to do this is to reset your backup plan:
FAQ: How to reset backup plan settings (single PC)Then perform the new full backup.
Reset the plan again, and perform a new backup on the original drive. it will be a full backup. After it completes, if you desire, you can then delete the old backup from the drive.
FAQ: How to Delete Backups-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator