Hi @jgirvine,
I just had that happen a couple months ago, and I had 250+ devices to move. That was not a fun experience, but manageable.
First thing to know is that you will not lose your smartapps or custom DH’s. You will have to manually move devices. Here’s my experience:
In my case my old hub was still online and excluding devices was easier. In your situation, you’ll have to perform a General Exclusion via the app for each zwave device (zigbee will be a lot easier to move). Just make sure when excluding your zwave device that nobody else in your house is using a zwave device, like a switch, that toggling causes it to be included/excluded from a hub. Using a minimote to exclude zwave devices can also be done.
When referencing the post I linked above, I think many of my steps apply to you and can help with your migration. I also included some lessons learned, with the most important one being patience.