Unfortunately there is no way to migrate messages between Cisco Unity Express and Cisco Unity (or vice-versa). What I would do is set up both concurrently. Then notify people in advance of the cutover. You can then also tell them that they will be able to retrieve old voicemails for a week (or whatever) using a different number (i.e. you'll change the voicemail number for Unity Express so that all new messages and the Messages button go to Cisco Unity, but you can still dial & log into CUE using a different number). That way there's a somewhat graceful transition.
