UPDATE Re: Zpool - can I go from mirror to raidz in place? Or best way to move?

It does appear that with this configuration  and zfs boot mirror there
was no smooth way to reconfigure onto 72G disks from a 146G mirrored
root.
We sprung for a replacement disk.

Thank you all  for  your help.

        NAME          STATE     READ WRITE CKSUM
>        rpool         DEGRADED     0     0     0
>          mirror-0    DEGRADED     0     0     0
>            c0t1d0s0  REMOVED      0     0     0
>            c0t0d0s0  ONLINE       0     0     0
>
>
> c0t1d0: configured with capacity of 136.71GB
>
>
> AVAILABLE DISK SELECTIONS:
>       0. c0t0d0 <SUN146G cyl 14087 alt 2 hd 24 sec 848>
>          /pci@780/pci@0/pci@9/scsi@0/sd@0,0
>       1. c0t1d0 <SUN146G cyl 14087 alt 2 hd 24 sec 848>
>          /pci@780/pci@0/pci@9/scsi@0/sd@1,0
>       2. c0t2d0 <SUN72G cyl 14087 alt 2 hd 24 sec 424>
>          /pci@780/pci@0/pci@9/scsi@0/sd@2,0
>       3. c0t3d0 <SUN72G cyl 14087 alt 2 hd 24 sec 424>
>          /pci@780/pci@0/pci@9/scsi@0/sd@3,0