I am aware that they treated Cambodians in Phnom Penh worse after they marched into the capital on April 17, 1975. From what I remember from reading a book on Pol Pot, this was because these were the last Cambodians to "join" the revolution. However, during the war, I'm not sure if Cambodians in Khmer Rouge controlled areas were treated the same (i.e., evacuated from urban areas and forced to work in rural communes, brutal treatment from the Khmer Rouge, etc.) after April 17.
The Khmer Rouge's brutality was established from the very beginning, and the treatment toward urban Cambodians was the same even before Phnom Penh fell.
The capture of the small city of Odongk in March 1974 was a early example of what would come later. The city was destroyed, and its 20,000 inhabitants were forced into the countryside.