Firstclasser careless yes, idiot no!
You need to examine the fare and change rules that were sent with the original confirmation to see what you can do.
QR are not known to be very accommodating with regard to itinerary changes outside of the fare rules and certainly not where fare differences are involved. So it all depends upon the change fees/Fare rules for your ticket.
The change fees are listed in the fare rules there. To this you may have to add the additional cost of fare differences. So all in all it could be quite expensive.
You need to talk to your local QR office to see if anything can be done. See
http://www.qatarairways.com/dk/en/contact-us.page for details.