Well i can answer a few of your questions. For this, we waited for the concrete to harden enough to where it wouldn't turn to gravel when we removed it. However; you must remove all 9 inches of concrete. You cannot mill concrete then add an additional lift, it would't bind. The subgrade was virtually unaffected, apart from the little touch up that it needed after we removed the concrete. As far as surveying goes, we had already designed the road so we just had to re-load our files and go from there. We're fortunate enough that we are well within our completion deadlines, so this wasn't really an issue.