If you vape 12mg on lets say a 1.2 ohm cartomizer then you switch one day 12 mg on a .5 ohm rda or sub tank your nic intake will be much higher because if nothing else the volume of vapor also if you go from mouth hit to lung hit IMO you get the nic faster so you can overdose causing nausea and or a light headed feeling.
How can your body reject something you put there its to late you cant throw it up or anything?
Look at it like eating. You can eat and eat but once your body feels full you will stop. If you don't you will make yourself sick. Ever said if I eat one more bite I'm going to be sick. Same with nic intake. Once your body gets enough you will put your apv down until you feel like you need more nic. If you don't you will feel sick or nauseous not necessarily throw up. When I say overdose I mean overdose to the point of needing medical attention. As far as your device change it depends on your intake. With the .5 ohm coil you will get more nic hence you will not have to vape as much to get that "fix" as you would with the cartomizer allowing you the "chain vape" more.