Map it out in Excel, column A is old number, column B is the new number.  Hopefully, you are keeping the first two segments in place (expanding one or two?) and adding two more segments to the right.  For some reason that I don't make sense of, you can only add segments to the right (not in the middle of).  Define/set up your segments via sub account maintenance.  Use your excel spreadsheet as source for the VI job to import into account maintenance.  Practice in a test company until comfortable