MrDooog welcome to the QR forum.
As you are probably aware all aircraft have to undergo safety ans maintenance checks at regular intervals Many of them require the aircraft to be taken out of service for the duration. This can vary between hours and several weeks. QR have a severe shortage of aircraft fitted with Qsuites and they are in constant use. Thus the aircraft has to be replaced with a non Qsuite aircraft.
If you look at http://www.qrcargo.com/checkrouting you can see the scheduled aircraft type for your flight. Do take into account that due to the nature of its information is loses accuracy beyond 5-6 weeks..
The situation will change as more Qsuite aircraft are delivered.