Extending root is I believe a maintainence mode job. Less i'm mistaken it has to be contiguous, so you would have to boot into maint mode and juggle things around. There is a much better way.
1. Back up the entire system
2. Create ignite make_recovery tape, use the -i option
3. Boot off the tape select advanced install
4. Edit filesystem sizes as required
5. Let machine rebuild off tape
6. re-mirror
Beware that this will only backup (by default anyway) vg00, so do not touch anything on other disk. (Hence the reason for a full backup, just in case).
Martin
Martin