1001 and 1002 can stay unchanged. 1003 - 1005 need to be incremented, and the new posting becomes 1003. The root stays at 2000. This scheme satisfies both of the final constraints because arbitrary insertion is accommodated and because any number of levels of followup is allowed as well. At least, any number of followups that fit within the range of 1000 numbers.
Nested followups share the range of numbers with any other followups.
Since the renumbering is localized to just the region of numbers assigned to a single root, this minimizes the impact. Since just the followups between the new posting and the root must be visited, this reduces the impact further. All ancestors of a new posting must be visited, to update their count of descendents. We do this by moving back from the parent to the root. All of the postings between the root and the new posting must have their sequence numbers incremented.