No personal experience, but I have read posts here where people have gotten very YMMV responses to calls about QR driven schedule changes.
I too would be concerned about waiting until travel had started. At the very least, request that someone annotate your reservation now with a note to say that you did not accept the changes - as that might help if you hold off until you are in DOH to get the change made.
Post back with how you get on,
Following your advice - I gave QR another ring and this time with specific instructions on what flights I want to change (last time I asked what my options were) and the phone rep was great. When she went to the help desk to work on making the changes she came back to me and asked me did I accept the new schedule changes because the Colombo office who affirmed the new schedule on their own discretion indicated that I did.
Made it clear I did not accept any change and the only communication I received was an Amadeus email from QR with the new schedule several days ago. She said they have to contact the Colombo office to have them confirm that I did not accept the change and if I did supply evidence. Once the CMB office confirms that I have not formally accepted the change then they can proceed with making a free change.
She added detailed notes to my PNR and told me to call back in 24 hours (tomorrow morning).
Will relay back my results tomorrow.