Brian,
The most efficient path is to use COBRAS. I have a number of posts explaining the overall process. At the highest-level, this allows you to build out the new Unity 8x environment, get it integrated with AD/Exchange, and then build out configurations that don't migrate over (there are a few which are highlighted in the COBRAS help). From there, you migrate data over from the 4x system. There may or may not be a few steps along the way but they are outlined in numerous posts I've contributed to regarding COBRAS and also in the COBRAS Help which is located at www.ciscounitytools.com.
Hailey
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