I successfully shifted the data from the 18s to the 73s today by doing this:

Warm inserted both new 73's after reading the PERC FM.
Set 1st 73 as Hot Spare.
Forced 1st 18 off line.
Let the system rebuild RAID on 73.
Put 2nd 73 online as Hot Spare.
Forced 2nd 18 offline.
Let mirror rebuild again.
Prepared both 18's for removal & removed them after they spun down.

Bounced on the OS (w2k Server SP2) for a bit.

Rebooted, crossed fingers and was very happy to see the OS come up no problem.

The mirror regens took only about 10 minutes each and the OS didn't burp in any way.

So now I have my two 18's sitting on my desk, which will probably go back in tomorrow. The PERC console is a horrendous circa 1996 interface. I don't think it supports extending the logical partitions. The bios version may, but I didn't see anything in the manual and I'm running current (ca.2001) firmware on the PERC. We'll see. I'm going to try very hard to avoid the 'restore system from tape onto fresh partitions' routine. I do not trust tapes for the OS stuff even though I use Veritas BENT 8.(?) That and they are ungodly slow. We'll see.

-Zeke