no.. media sets define your tapes and drives/ overwrite settings etc..
BackupExec differential jobs are defined as having changed based on one of two methods, (your choice set in the job properties), either by the archive bit or by the time/date stamp of the file. (use archive bit its easier)
Under the Back Job properties, you'd change your backup method from 'Full - reset archive bit' to 'differential - changed files'
thats it.. pretty simple