Another vote for ripping the bandaid off and moving over. I had a wonderful plan for my migration, but ended up moving over all at once, and it ended up being the best approach for me. If you come across in pieces you can end up w/a weak mesh on both hubs, not good.
I think this thread would be a good read before you start:
This post in particular from above:
I have a mixture of older Z-Wave switches and plugs (and water valve & Schlage lock), newer Z-Wave+ switches and plugs, and a few battery powered Z-Wave contact sensors and smoke/CO alarms. The Z-Wave vs. Z-Wave+ devices are mostly randomly distributed around the house. The rest is Zigbee contact, motion, and leak sensors.
I didn't replace any of my Z-Wave devices, and don't think you need to replace a bunch of Z-Wave devices with Z-Wave+ to move to Hubitat. YMMV, and depends on how things are distributed around your house, where your hub is, etc.
Try to get your hub in central location in your home. My experience w/HE is that it has seemed to be more sensitive to hub location than my ST hub was.
Definitely work from your hub outwards, starting w/powered devices first, then add battery devices.
Even if the device your moving over has only ever been on ST, you may have a better experience if you do an exclude on the HE hub for the devices before you try to add them to HE. Turning off the ST hub before including on HE certainly can't hurt, and may help.
The current FW version (2.2.3.145) is much improved over the version that I used to move to HE, so you should be in better shape for a successful transition.
Most importantly, ask questions if you have issues...anything you run into will likely have been seen and dealt with by one of our helpful forum members.
