I’m seeing some confusion here because “wired“ sensors and “hardwired“ sensors actually mean two different things in this context.
A smart sensor needs to be able to do two different things: recognize a change in state, such as open instead of closed, and communicate that state to the controller.
With a wireless sensor, there’s a regular sensor inside the device, plus there is a radio which allows it to communicate wirelessly with its controller, in this case the smartthings hub. By convention, this is also a battery powered sensor. The battery is used both to power the part of the sensor that recognizes the state change and to power the radio and the radio actually uses quite a bit of power.
Next we have soft wired, or more accurately mains powered, smart sensors. They still recognize the State change, they still communicate by radio to the controller, but instead of getting their power from the battery, they are plugged into the mains. There are some zwave and zigbee sensors which can operate either from mains power or from a battery.
Note that there are some companies that will call these “wireless powered sensors,” because to them the “wireless“ means radio communication.
Finally, we have hardwired sensors. This actually doesn’t have anything to do with how the sensor is powered because often the sensors are not powered at all!. This has to do with how the sensor communicates with that controller, which is over a traveler wire very much the way the auxiliary switch works in a regular nonnetworked three-way set up. The sensor is wired to the panel.
There are different ways of wiring them, but it’s not uncommon To just have a little current that runs through them and if the contact is open then the current doesn’t come through and it is the control panel, not the sensor, that recognizes the state change. So this kind of hardwired sensor is really dumb. But they can also be used with many different control panels because they aren’t Formatting messages at all. So sometimes these are called “universal hardwired sensors.“
Anyway…While it is possible to have a smart hardwired sensor, that is a sensor with its own brain that can recognize its own state change and do something about it, you don’t usually see those in low-cost residential systems. you do see them on things like banks systems were you want a lot of redundancy and they can function as a kind of tamper alert.
So it is even possible to have a “hardwired wireless batterypowered sensor” because hardwired refers to how the sensor communicates to the controller, wireless refers to a secondary communication method, and battery powered refers to where the sensor gets the power to run the radio for the secondary communication method. But again, you don’t see those in Low end residential systems. But they do exist.
Back to the question: does it make sense to swap out one sensor at a time?
OK, for the purposes of low-cost residential systems, “hardwired“ almost always means a dumb sensor with the physical wire connecting it to the panel and a tiny bit of current going through that circuit. No radio. No power required for the sensor itself – – sensor itself doesn’t even know if it’s open or closed. It’s the panel that determines that based on the current received through the traveler wire. Sometimes the sensors are a little smarter than that, but they still don’t have radios. So whatever power they need to run their brain they just get from what is called the “residual power“ on the wire that they are wired to. They don’t need a battery or a plug.
Back to the OP’s question:
If you just hate batteries, you can convert your standard wireless sensor so that it runs off mains power instead of off a battery. Lots of people have done that. But you can’t swap this in for a standard hardwired sensor because they just Communicate completely differently To the panel.
It’s not impossible that you could rig up a kind of a Rube Goldberg thing with dry contacts so that when the smart sensor detected something you sent a pulse message on a traveler wire to the panel, but it wouldn’t really make sense to do it that way.
It’s also really important to understand that your typical hardwired sensor is very small, very cheap, and carrying almost no power. These are often the size of a quarter.
But your typical “wireless“ sensor is also a smart sensor. It has to have room for a brain that recognizes the state change, for radio to communicate that state change, and for a power mechanism, whether it’s a battery or a plug, to power both the brain and the radio. That’s why the sensors are commonly the length of a finger. And generally cost much more than hardwired sensor, because they are a much more complex device.
You put all of that together and that’s why you don’t just swap out one sensor at a time in a hardwired system.
If you use something like konnected, what you are doing is swapping out the brain that monitors the dumb hardwired sensors.
If you start adding wireless sensors, then you are communicating via radio, not via traveler wire, usually directly to the hub.
So it’s an apples and oranges thing. All of these kinds of sensors are useful for different purposes. But once you have some hardwired sensors, you’ll generally only use the wireless sensors for specific locations where for whatever reason you don’t want to run wire. You can’t take a string of six hardwired sensors cut out sensor number four and replace it with a smart sensor. It just isn’t going to work that way.
Oh, and it used to be the “wired” was used interchangeably with “soft wired.” You had battery operated sensors, wired (plug-in) sensors, or hardwired sensors.
But as there have gotten to be more and more wireless sensor systems out there, more installers are starting to present the choice as “wired” (meaning hardwired) versus “wireless” (meaning either plug-in or battery operated, but communicating via radio ). So you do find the same terms being used different ways by different companies or even different installers for the same company. As long as you understand that part of the issue is the communication method and part of the issue is the power method, and that sensors that have radios need more power, you should be able to sort it all out.
I hope that helped. 
@tgauchat @heythisisnate
