Need to convert from rapid-pvst to mstp

I have a network with a Cisco 6509 sitting at the core and fiber connections from there to 25ish IDF's that contain a mix of 3560's and 2960's. This whole setup is running in rapid-pvst. The 6509 is configured as the spanning tree root. The 6509 is going to be replaced with a stack of Meraki 425's and the edge switches will be replaced with MS350's shortly after that. Since Meraki cannot be the stp root on a Cisco network running rapid-pvst I need to convert to mstp. I have done this on a test switch connected to the Meraki and it worked. I didn't do any vlan to instance mapping, just changed the spanning tree mode. Is there really anything else I need to do in my case? In the test, before switching to mstp the 425's were not root even through they are configured to always be root. Once I changed the mode to mstp, it put vlans 1-4096 into the default (mst0) instance and the Cisco switch showed that the Meraki was root over that instance. I know I have to convert starting at the core (and need to set priority at the core) and then work my way out from there. It seems pretty easy so I think I must be missing something.