Adding MST Instances to an existing configuration

[prelude]

I have two core switches and 40 aggregation switches running MST, with one instance (0). The two core switches are the secondary and primary root for the MST instance. I have some PVST only switches in the mix with the root set to the same as the MST0 instance. What is the best way to migrate some VLANs off the 0 instance and into another instance? Should I just create another instance on all the switches, then migrate the VLANs I want to move into it? Should I start moving the VLANs to the new instance on the root switches first, then go for the aggregation switches (I realize that as soon as I move any switch into the new instance it will be the root, but should I start off that way for any reason).

I want to minimize downtime while this is done. Does anyone have any guess as to how much downtime this would cause and at what points in the transition would I have downtime?

Thanks

andrew

[wong34539]

Any MST configuration involving a large number of either existing or new logical VLAN ports should be completed during a maintenance window because the complete MST database gets reinitialized for any incremental change (such as adding new VLANs to instances or moving VLANs across instances).

For more information : http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/122sx/swcg/spantree.htm#wp1061736

[prelude]

So far this hasn't been a problem, but I'm wondering what the best practice would be for adding a new instance and then adding vlans in those instances? There must be an order of performing these tasks that is less disruptive than others. Really what i would like to know, is that while I am transitioning one vlan over to a new instance, will the entire network continue to work if the topology stays the same (i.e. a vlan split between two instances on a variety of switches).