> Adding LTO5 drives shouldn't cause any trouble. They'll be able to
> read/write your current LTO4 tapes.
> The fun will begin once you add LTO5 tapes in the library.
> These tapes cannot be used by the LTO4 drives so, you'll likely have to
> create a different pool form them and select only the LTO5 drives for 
that
> pool.

I had to do that when I had a mix of SDLT320 and SDLT600 tapes. I ended up 
with 2 different pools ... confusing.

> You can avoid that by replacing your LTO4 drives with LTO5 ones.

That can get expensive. :-)