To avoid rewiring stay with the FireX brand. FireX makes Heat/Smoke/CO and combination sensors that are compatible with each other which is very important. While all mains powered detectors will connect to Line and Neutral (for the 120V AC supply), the signal on the 3rd wire differs between manufacturers (and sometimes even within different product lines of the same manufacturer). There are Kiddie sensors that are compatible with the FireX sensors and there are even UL listed adapters so that you can keep not only the same wiring but even reuse the existing connectors. However that does require careful checking of the compatibility charts (not all combinations that are functional from a purely electrical standpoint have been tested and certified).
If your original installation was done properly you can simply exchange each sensor/detector with the same kind that is currently installed (more likely its successor model). However this is also an opportunity to improve protection by upgrading single purpose sensors with combination sensors that will detect multiple threats (such as replacing a simple smoke detector with a smoke/CO combination sensor in locations where that makes sense).
Beware of people making blanket statements such as "all ionization smoke detectors should be replaced with photoelectric sensors" (this nonsense if sometimes propagated by officials who really should know better). There are scenarios were one is more likely then the other to raise the alarm faster (and when it comes to a house fire every single second counts!) but generally a house fire produces so much heat and dense smoke very quickly that there isn't much difference. Unless the sensor/detector is in a location where the ionization sensor is likely to cause false alarms (e.g. water vapor outside bathroom) I would suggest to go with the suggestion from jack b
and spend a little more on a combination photoelectric and ionization sensor to be sure of the quickest possible alarm time.
Another reason to stay with the FireX brand (or properly listed compatible Kiddie sensors) is that it allows you to upgrade one sensor at a time (if either time or money are in short supply and you want to avoid replacing all sensors at once).--
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