This is what the help says:
"...every new differential backup is compared with the base archive. So it contains all the files that were changed/created comparing to the base archive, even if these files were included in the previous differential backup already. Therefore you don't have to store all the previous differential backups to be able to restore the last one."
Therefore if it it a differential backup (and not an incremental backup) that seems to explicitly say that those old differential backups can be deleted.