Yes, that is always a gramatically correct (but unhelpful) answer to any question that begins with "Does anyone know...." regardless of the topic.

There are always aircraft substitutions and changes, regardless of the airline; it's not just QR, and it's not just the route. Try flying CX between HKG and BKK in either direction! There are numerous possible reasons, including everything from scheduled maintenance to an aircraft charter to expected new aircraft delivery to delayed new aircraft delivery. If you really want to get into the details, try using one of the available tools on the internet to track aircraft by tail number. You'll then be able to take a particular flight and find out where the aircraft came from, where it goes next, etc., and perhaps start to get some idea of the rotation patterns which affect that flight in normal operations. That may (or may not) give you the specific information you're looking for, but for a travel geek like yourself (and me, too!), it will definitely be interesting!