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

We have a T2000 production server with two 146G disks in a mirror in
an rpool, one of which has gone bad. The system has two unused 72G
disks, see below.
We've got a lot of spare used 72G disks in house and no spare 146G
disks, and we are not eager to buy replacement T2k parts at this stage
of the game.

What I'm wondering is, can I break the mirror, and replace a 146G disk
with two 72G disks? or should I break the mirror, replace the bad
drive with a 72G and then make a raid-z out of those three and create
a new BE on that rpool? Is that the cleanest way to do this?


 zpool status
  pool: rpool
 state: DEGRADED
status: One or more devices has been removed by the administrator.
        Sufficient replicas exist for the pool to continue functioning in a
        degraded state.
action: Online the device using 'zpool online' or replace the device with
        'zpool replace'.
 scrub: none requested
config:

        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

errors: No known data errors
# format
Searching for disks...done

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