here’s how we pulled off a successful migration (at least in my opinion):

• kept vm pilot to old unity system active so old messages could be accessed
• created a new vm pilot for unity connection
• disabled MWI extensions on old unity system
• manual MWI resync on unity connection
• changed default vm pilot in call manager to the unity connection pilot
• changed our external vm access number’s transform mask to the unity connection pilot
• created a forwarded routing rule for our greetings administrator extension in unity connection to route those calls to the greetings administrator conversation

unified messaging (voicemail/email integration) is explained in detail in the “unified messaging guide for cisco unity connection”