Always get the new PV, dump the program in and make sure that it completes the validation process before RIPPING out the old!
You can always power it up remotely and plug in your comms as required....before you rip out the old.
Just from experience....
But you shouldn't have to worry about anything.
The 900 was discontinued in 2001, and so is the 1000 now I believe.
The Rockwell group wants to migrate you to the new PV+ as soon as possible.
Don't do it unless you have to.