Change Independent to Incremental back ups

we set up Norton Ghost 14.0 to do Independent backups, Hard drive is 250 gig and full.  Then went back and deleted all but first and last recovery pts and set to continue to delete extra after back up is done.  I thought I was doing Incremental and tried to change back to it or start another back up and delete the old but cant.  What is best?  Thanks, Terri