Implement rotating NTBackup on NAS

Hello, we have SBS203 and HP DAT 160 GB tape. We have used NTBackup for daily full backups (Monday-Friday). We had 10 tapes to circulate (meaning we were able to restore data from a tape which is 14 days old) - SCENARIO 1.

Recently we have reached our data tape limitation 160 GB and had to find out a solution for more data to backup. Our management decided to buy new QNAP_NAS_TS-459-Pro-II with 4x1.5 TB discs - SCENARIO 2.

I configurated a backup wizard (NTBackup) to create a full backup to NAS share folder. The only difference between SCENARIO 1 and SCENARIO 2 is that SCENARIO 2 backup is stored to NAS and NOT on tapes. All other backup configuration in SCENARIO 2 remains the same as in SCENARIO 1.

The first backup on SCENARIO 2 has been created successfully. However, each day the backup file (*.bkf) is overwritten with the new one. So to my knowledge, it will not be possible to restore data from older backups as it was possible when using a few days old data stored on tapes.

The question is, how to implement backup SCENARIO 1 where backup location is on NAS and that NTBackup will overwrite only those *.bkf files which are older than 14 days?

Thanks in advice.