>That is not true, I had to relabel a tape this morning.

You need to tell us why.  It is very rare to have to relabel tapes.

>What you do is...
>
>check the questionable tape out and then run the following command
>
>Label libv lto_lib search=bulk labels=bar checkin=private overwrite=no
>
>This will re-label the tape and keep the data on it ...

As Wanda says:  NO!!  NO!!

Relabeling any tape fully resets the volume, as in All Gone!
Tape is not a random-access medium: you can't update in the midst of it.

If you need corroboration, refer to documentation of the dsmlabel command:

  Attention: Data is lost when relabeling. Before relabeling a Tivoli Storage
             Manager volume, you can either move the data (MOVE DATA command) or
             delete the contents of the volume (DELETE VOLUME command with
             DISCARDDATA=YES).

  Richard Sims