Can I replace an electric smoke detector with a combined smoke and carbon monoxide (CO) detector?
In our area, local codes made at least one CO detectors mandatory in all condos and rental units.  I have had to replace dozens of standard smoke detectors with combo units in hard wired/battery back-up systems.  Most every manufacturer makes a CO/Smoke unit that will plug in directly to the existing system. A hard wired replacement is far better than a stand alone battery operated unit, as it will alarm all units in the system if there is a problem.  

Having the system monitored or not monitored makes absolutely no difference on how the system works inside the house. All monitored systems will work fine in the house even if the monitoring system is off.