I plan to visit tourist spots - Siem Reap, Sihanoukville, and live for a couple of days in Phnom Penh. As far as I remember, they accept dollars in Siem Reap without fussing about exchanging - the only question is whether they accept "old" ones normally or with some discount?
Siem Reap. Recently at ABA Bank, they simply replaced the old ones with new ones without any additional payments.