Great question — answering as the UphillTech team:
I have already bought the ticket. Unfortunately Lufthansa wasn't offering the connection time I wanted to book and Air China's website had technical problems. I bought from kissandfly.de and I couldn't book without providing the passport information. However, I can change the passport information on Lufthansa's booking page. Therefore, I will be able to change my passport information after I receive my passport and before Check-in. I can change both the expiration date and (if it would matter) also the passport number.
Full answer by André: https://travel.stackexchange.com/a/203621 (CC BY-SA 4.0).
Need it done for your business? UphillTech.