Upgrades from EL7 to EL8 are supported, though.
For Container Linux/Fedora CoreOS, in-place updates don't make much sense. Those are intended as immutable OS images. They get their configuration from a remote source specified on the kernel command line.
The migration work is dealing with changes in the configuration language.
Yes, that's how all installations work for CoreOS Container Linux. The problem is the day when the autoupdate of the current distro stops (no more new images, and that's supposed to happen soonish, although no date has been communicated) your old configuration language will stop to work (unless you are happy enough not to used anything on the list of incompatibilities, which is pretty unlikely for non-trivial installations). And then you need to port and test.
In my experience coding the configuration takes at least 3 times as long as installing in a traditional way. That is a worthwile investment if you want to be sure what you have installed and want to be able to start identical machines later. But if the investment is broken by commercial/political decisions that's frustrating.
Well, I am not a paying customer, so I cannot really complain. I just have to decide whether I am happy with the flatcar fork or whether I port my configuration to Fedora CoreOS. At the moment they have no complete documentation AFAIK, So it's difficult to make a decision.
We also released the update service as an open source project, Nebraska (https://github.com/kinvolk/nebraska). That was something that was closed source with CoreOS. This allows you to be in control of your updates.