Thank you for the reply, can we have documents to perform this ? 

Sure you can. You can either clone the backups to a new pool with media in the new jbox or you can write-protect the old media and use the same pool.

Sorry - these are standard NW operation. AFAIR there is no specific document for this purpose.

I agree, cloning (or maybe even staging as then the old tape is also freed up right away) is NW basics 101.

You might not want to move the old tapes into the new library. It can work as LTO supports (at least for reading) up to two generations lower. So LTO4 should still be readable on LTO6 drives.

Cloning (or maybe better staging) the data, let's you get rid of them old tapes. Only needs a NW storage node to have access to tape drives on both old and new tape library.

Fellow lurker on this community crazyrov, has stated some info about cloning (however in this case when using datadomain as backup medium)

https://www.youtube.com/watch?v=ZgjOMKHM-D0

https://nsrd.info/blog/2011/03/21/basics-cloning-vs-staging/

So you better dive into that NW manual to understand them cloning basics.