Can I replace my existing Fire or CO detectors with new wireless ones?

The easy answer is yes. If your smoke detectors are currently battery powered, there will be no problem to replace the detectors with new wireless battery powered sensors. If you have hardwired detectors, the current wiring will have to be terminated accordingly and the smoke detectors will then have to be mounted on top. There will be no issues replacing a hardwired detector, it will just have to be terminated accordingly.