I changed the LHR-ZRH leg of a LHR-ZRH-JFK ticket this morning and checkmytrip shows the new flight details with the same ticket number.
I called LX to select seats and they too confirmed that the ticket number remains the same.
I have only had a new ticket number where there are major changes. A previous trip, LGA-BOS-(ZRH)-LHR was changed to JFK-(ZRH)-LHR and that triggered a new ticket number