Here is what I would do (CLI - manual method):
Remove the old vx objects totally from the cfg (clean it out):
If disabled (vxedit –g rootdg –r rm rootdg01-0x)
if active (vxplex –o rm dis rootdg01-0x)
same as above for remaining plexes (see vxprint -thr & vxdisk list output)
vxdg –g rootdg rmdisk xxxxxxxxx
/etc/vx/bin/vxdiskunsetup –C cxtxdx
vxdisk rm cxtxdx

Run format - zero all partitions except slice 2
Label the new disk under format
run vxdctl enable

Find privite region lenght on existing disk (to match)
run vxprint -thr
TY NAME ASSOC KSTATE LENGTH
sd rootxxxxxxx - ENABLED 2743
(alt command) vxprint -thr | grep sliced
DM NAME DEVICE TYPE PRIVLEN PUBLEN STATE
dm rootmirror c1t1d0s2 sliced 2743 71681511 -

/etc/vx/bin/vxdisksetup –i cxtxdx puboffset=2744 (“rootdiskPriv” length + 1)
Note: Substitite the option old_layout for puboffset above for patch level 1 and above. (output: modinfo | grep vx = 3.2t_p1+)
So the command above would be (without puboffset statement):
vxdisksetup -i cxtxdx old_layout
vxdg –g rootdg adddisk rootdisk=cxtxdx
/etc/vx/bin/vxrootmir rootdisk
vxassist mirror swapvol rootdisk
vxassist mirror opt rootdisk
vxassist mirror usr rootdisk

I know that this works as I have used it many times in my env.
Please don't take my word for this - you need to test in your env.
Test on a test system by pulling a mirrored disk on a test box.
good luck