I've replaced a Kasa wifi smart plug with a Z-Wave plug. The new device is working fine, but when I use the "Swap Apps Device" function, the Kasa plug is not available to select as the old device!
This is normal behavior. They are child devices of the Kasa integration app, and child devices are intentionally not supported by the Swap Apps Device tool (the parent may have specific expectations about the child device that would break if it were taken out or swapped), as noted in the docs: