Since a Windows installation is mainly just an unzip and registration of a service file, you could probable do a side-by-side install for OH 3 and do a fresh configuration on OH 3. The main restriction will be you won’t be able to run both your production OH 2 and the new OH 3 at the same time. But it’s easy enough to shutdown OH 2, manually start OH 3 and do your work.
Once OH 3 is where you want it, you can modify the service file to point at the new OH 3 instance.
Anyway, I can’t comment on the Windows upgrade process. I know early on the service file wasn’t quite right in the docs for OH 3 but I think that has been fixed.
I would wait until after the upgrade. I believe the script posted at Migrate your existing persistence data to InfluxDB should be generic enough to work for this with only minor modifications.
I just cut my losses and started over. The advantage there was as I was rebuilding from scratch I was able to change the names of some of my Items. To migrate the data you must keep the same Item names.
I started importing the Items from my .items files. But I also wanted to use the model and it turned out to be a lot of work to retrofit my Items to the model. So instead I rediscovered and created my Things and then used the “Create Equipment from Thing” option to recreate the Items. For me this was twice as fast as trying to rework my existing Items into the model.
Definitely have at least a quick skim of the Getting Started Tutorial.
No real advice but a quick tip. You can exclude a device from a different controller that it was included on. For example, you could use the Aeotec to exclude a device from the Z-stick and then include it on the Aeotec. That might make your migration between the two a little faster.