>At the moment we use about 900 J-cartridges in our library. Is it possible
>to checkin K-cartridges and use them besides the J-cartridges.

To bolster Jim Sporer's reply: yes, it is commonly done.  We've been doing
it for quite a while, largely treating the J tapes a "prior generation" and
so best suited to lesser roles such as store-full-tapes-outside-library,
given that lesser capacity makes for the use of more of the limited cells
in the library.  Output from the 'mtlib' command can distinguish between
the two.  I have a handy-dandy perl program which invokes the mtlib command
via more natural expressions.  If interested, write to me directly (rbs AT bu 
DOT edu)
and I'll send you a copy.

  Richard Sims, BU