Per the Unity Connection Migration guide(s) you should not do migration operations during business hours ""
Caution 
Do not migrate subscribers during normal business hours.
http://www.cisco.com/en/US/docs/voice_ip_comm/connection/8x/upgrade/guide/8xcucrug024.html
However, COBRAS depending on its application could be run during the day. Of course, this all depends on the system. Clearly you don't want to be running any COBRAS operations on a Connection cluster during busy hour with 20K subscribers.
