Changing Retention levels on Tape

Hi.

We are using Symantec Netbackup 7.5 on Redhat Linux with standalone tape drive .  I have created backup policies with retention level= 2 months, and all backup's have been successfully completed on allocated tape A0005.

Now, I have to change the retention levels of all backup policy to 1 year, is there any way that I can use the same tape(A0005).  Currently the backup fails when I change the retention level, it's asking for new tape .

Can I manually expire the old images on the tape and delete it.

Thank You