Is this possible …
Replace an existing Z-Wave device with a different type of device (then change device handler to an appropriate one for the new device). Basically I am pulling some smart switches and replacing with a different brand.
My thought process is to avoid redoing automations which would be necessary if I simply remove the old devices.
The brand shouldn’t matter but you are only supposed to use the zwave replace utility with a device that will selfidentify as the same device class and which uses the same Z wave commands. Otherwise some of the automated network functions like routing and status checks won’t work correctly.
I have no idea how the SmartThings platform ability to switch from one DTH to another affects that whole process.
Okay JD. I think that’s enough for me to just play safe and do removes on the old devices.
Thanks
Oh come on, swap one and see what happens. For science! 
Said no consumer-facing engineer, ever. 
Um, no. lol.
If I do I should wait for a Friday right. That’s usually when ST likes to take a nosedive.
I’ve been doing this to upgrade old GE Z-Wave switches that drop off the network with new Z-Wave Plus ones. Seems to work without issue, but as @JDRoberts said, while you can change the DTH, SmartThings doesn’t seem to update any of the network / device layer stuff, and the Raw Description and Data describing the device remain the same even after a replace.
I had no problems replacing any dimmers / switches with this method, but I wasn’t able to use replace with a new lock. While the lock would pair and unlock/lock without issue, it wouldn’t accept code programming.
In another thread I asked if it was possible to refresh the network / device layer stuff on a replaced device, but it doesn’t seem doable. Or potentially it does refresh, but the new info doesn’t get copied to whatever database the IDE pulls from to display the info. Z-Wave Tweaker does show the correct info when it queries the device, so it is possible it is just a display issue. It would be nice to have a comment from ST.
Example. Two identical GE/Jasco Z-Wave Plus switches.
This one was REPLACED from an old GE/Jasco switch:
Where as this one is a NEW install of the same model Z-Wave Plus switch:
