There's been many occasions where I've wanted / needed to move the vol0 to somewhere else. Just last week I had a customer that wanted to destroy the root aggregate and move some disks around, but they'd already setup the filer and didn't want to lose this.
The process is actually very simple!
Renaming the vol0 is exceptionally simple and needs no downtime.
vol rename vol0 new_name
And it'll be done live! You might want to re-export your NFS exports, double check the CIFS shares and update any SnapMirror / SnapVault, but from the filers point of view, all is fine!
To change the filers root volume this is also fairly simple. This can be really useful, so if you copy the vol0 to somewhere else, or you need to recover it from a SnapMirror destination, or you simply just want vol0 in a different aggregate.
First of all you'll need an extra root volume, this does need to be a complete and full system volume with OnTap and all the relevant config files already in it. The filer does a quick check, but obviously not a full boot test. Once you have this volume in place, you simply tell the filer this will be the new root volume
vol options vol_name root
And on boot this will become the new system volume. If you've moved aggregates, the root aggregate will also get updated.
If for whatever reason you manage to kill the filer by doing this (say you didn't actually have a full root volume that you told it to use), then go into maintenance mode and you can change the root volume back from within here (CTRL+C on boot, then option 5).