It depends on which flight the downgrade has occurred on.

If it is the FRA-DXB sector then there is an entitlement to the entire ticket price under the regulation. There is nothing which states it is a pro rated entitlement. The airline could rebook the person onto another service involuntarily and then the entitlement will be just the delay compensation should the delay be >3 hours

If it is any other sector, then the EU compensation does not apply


If the downgrade is on the FRA-DXb sector

At the moment, you complain to no one I would suggest if hoping for compensation.

If you are wating to travel in 1st class , then contact Qantas and ask about rebooking

Should you bee delayed or downgraded on the day, then lodge a claim against Emirates since it is the operating carrier and so liable under the regulation

If it is any oher sector

Contact Qantas for any fare difference refund that may be due and any compensation which the airline may be prepared to offer. The refund is likely to be close to zero though since there is likely little or no difference in the fare