[villagetinker]

OP, most detectors actually PLUG in at the box on the wall or ceiling. If you twist the existing detector, it will come off the wall, the connector will unplug from the back of the detector. If you get the same make and model, then you simply reverse the procedure. I would strongly suggest you replace ALL of the detectors at this time. Also, the detectors outside the bedrooms are typically CO2 detectors, make sure you get the same type for these. Any handyman will be able to help you if you do not want to climb the ladder, and electrician is not required.
If you decide to change brands, you may need to change the connector, which will require the power to be turned off and will require more time to remove the original connector and install the new connector. As far as I know you cannot mix and match different detectors.