duplicate job from different tape media

Hi, I have a backup on LTO-3 tape media and would like to duplicate the tape to LTO-6 media.

both tape librarys are connected and functioning on backup exec 2012. 

backup is on 000039L3, and would like to duplicate to 000024L6. When I right click on the tape backup and say: duplicate and choose the LTO6 drive, it asks me to put in 000019L3 tape in my LTO6 Library, which will not ready a LTO6 tape.

Whats the best way to do this?  The server that was backed up is no longer with us.

I want to toss our LTO3 Library and convert all my tapes to LTO6.

thank you for your help.