"At what elevation will all of the vertices end up when you convert a 3D polyline to a regular polyline, using the Drawing Cleanup tools?"
The answer is that all of the vertices will be moved to the Z-value of the FIRST vertex of the 3D polyline. For example, if you start with a 3D polyline that was drawn from "10,10,10" to "20,20,20", and then use the Map 3D Drawing Cleanup tools to convert that object to a regular (flat) polyline, then all of the vertices will end up at elevation 10.
For the complete tutorial, (and many others) follow this link...
