I currently have a (very stable) OH 2.5.9 system running on a RPi4. I also have a unused RPI3 laying around. After all the enthousiastis posts I want to check out OH3 too.
What would you guys advise? Copy current OH2 to the RPi 3 and start installing/ updating on the RPi4? This should become my main system in the future. Or should I start in the RPi3 and keep my current system as is? I can’t really see any big pro’s or con’s for each approach so I’m curious what you’d advise.
If you did that you could use the remoteopenhab binding in OH3 to access the OH2 Items to ease migration. I plan on something similar in my VM installation but have installed OH3 in a Docker container.
Milestone 4 is broken so either use Milestone 3, a snapshot, or wait for Milestone 5.
I use openhabian, it will even work on a Pi0 although the UI is a bit slow, I’d say a RPI4 is a bit over kill.
But not supported. I found a very small system taking up half the RAM on my Pi 3. The current openHABian recommendation is a 4GB or less Pi 4.
It was mentioned in the Virtual Meetup that there isn’t yet documentation for upgrading 2.5 to 3.0. So unless you want to contribute to that, you might be better off starting OH3 from scratch and copying over your rules. It’s a good opportunity to adapt to using MainUI, particularly with respect to items. And then you can just copy your rules over. As Bruce points out, you can even just use the remote binding to connect to your old system.
When I make the jump, I’ll probably just put a fresh OH3 SD card in my RPi4 to monkey around with it. And then I can just swap back to OH2.5 if I don’t get far enough. I live on my own, so I’m not inconveniencing anyone else if OH is offline for a few hours. 
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All RPi models are supported.
To use the 0 and 1 models is a bad idea though. Definitely the wrong place to save the money.
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Keep your current system working.
Stick OH3 on unused PI to play with.
OK, here’s what I’m gonna do.
First I’ll fool around with the RPi3 and a fresh OH3 install.
I like the idea of rpwong, but I’m running everything from a ssd.
As soon as I feel a little comfortable with OH3 I’ll start migrating. I’ll keep you posted.
Thanks for the advice all!
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Then where’s the fun in that ? OH releases are always due in December probably because migrating without the risk of breaking your heating system is only half the challenge 
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Then you need to release in July for those in Australia. They are in summer in Decemnber 
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Thanks it is hot here
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Do you spend Christmas on the sandy beach?
I worked with an Aussie for a while and I understand that was quite common.