I think you'll find you have to retain the existing Master/Media server infrastructure until all the Netbackup images have expired (depending on your retention periods)
So you'll need a new server for a start and possibly a bunch of new tapes to account for the unexpired images. This might mean a larger silo (or even an additional one unless you can share the current one between two backup servers) unless your present one has sufficient capacity etc etc etc.
Also, there is extra work in installing new client software, testing with (and I'm not even sure you can have BE and NBU clients installed together) the new software etc etc etc.
I would consider this a backward step, in all honesty.