I received exactly the opposite opinion from the CPT agent to that given to the OP. I am booking a DONE4 out of JNB and was told specifically that I could change the first flight date without any problem. In my case, I am departing on CX and managed to get CX to book a D class seat for me on my desired date when AA said there was no availability. I plan to get the ticket issued by AA tomorrow with an earlier departure date and then give the tiicket number to CX to keep my desired (later) date.