From my experience HW and breaking the mirror you loose all data.
Is there is slice big enogh you can use on the same disk?
or on an other disk that is outside this HW volume.
The last time i did it was upgrading the server vial live upgrade to an other slice on a different volume.
Then boot from that slice .
destroy the hw volume where the system was running. Then create a 3 way mirror to the 2 boot disks after the sync detach the current boot disk and reboot
Then reboot the server to make sure it boots from those disks.
As I sad, there is very bad configuration, from jumpstart configuration which I saw, there was small volume /var, swap and rest was dedicated to /. If there is way to shrink this slice (I know there is no way in UFS), it will be possible. Also we have only 2 disk configuration, so it means I see only one volume c1t0d0.
I'm just asking if there is some, non documented method to split this mirror because I also see this recommendetation.
Maybe there is possibility to personaly meet this server, detach for 2nd disk and connect it to 3.rd slot, we will became one way mirror and also, we will be able to see new disk (I hope), and this new disk we can use for live upgrade, after liveupgrade and everything working, destroy hw mirroring and create zfs mirror.
Another possiblity is use of SAN, but I'm not sure if we have FC card on all server in environment.