Hi, Page 861 explains the process in more detail [URL] You shouldn’t have downtime from turning it on, it creates a new instance of STP for each VLAN, but when you change master switches it probably will have some seconds of downtime while STP learns again.  
Hello, We have a N4032F Stack configured with RSTP. It has two uplink switches, VLAN 20 and 21 uplink to a Unifi switch and VLAN 200-210 uplink to a cisco switch. The stack is using RSTP and it elected the unifi switch as the root bridge since it has an STP priority of 4092 while the cisco switch has an STP priority of 8192. The stack also has vlan 400-450 internal to itself.  We are out of ports on the stack and are adding a nexus vpc setup. The nexus switches will uplink to the cisco and unifi the same way the dell stack does for vlans 20,21,200-210. However, for vlans 400-450 the nexus switches and dell stack will need to connect. The nexus switches will eventually become the root bridge for vlan 400-450, but need to be moved over one by one. So, I am hoping to enable RPVST on the dell stack without interupting RSTP, connect a trunk between the dell stack and nexus switches, and when ready to move a vlan specify a higher STP priority on the dell stack. That way the nexus switches have a lower stp priority and become root bridge.  The first step to enable RPVST is "spanning-tree mode rapid-pvst" after doing this, will this impact the current RSTP configuration? KB000120211 makes it seem like vlans can be added and removed from RPVST, once it has been enabled. However, I am unsure if enabling or configuring the vlan STP prioritoy as mentioned above will cause a disruption. If anything is unclear please let me know and I will correct! I appreciate any input, thanks!
Enable RPVST without interrupting RSTP?