I have the following option set on all of my backups to make sure that I don't end up with a problem with a missing full or differential on an error and be unable to restore.
Rerun Failed Levels = yes
I have noted that changing the fileset causes the next backup to be upgraded to a full, this makes sense. However yesterday I changed the time that my backups were scheduled and they got upgraded to full backups. That's annoying.
Jon