So, as CoreOS Container Linux has been end-of-life'd, I've switched both my Google Cloud VMs over to Flatcar Linux, using the in-place upgrade method (I want to avoid having to rebuild the base container if possible.)
The newer of the two has upgraded perfectly, but the older one has a failed unit on booting, which is related to the Google specific services in the /usr/share/oem directory.
Upon looking at those directories on both VMs, they are entirely different, even though both VMs were originally built using Google's official images for CoreOS. The newer one is using a RKT container called oem-gce (which doesn't exist on the older one), while the older one appears to have a bunch of local scripts including a static copy of python installed directly in the /usr/share/oem partition, and seems to be managed by a copy of system-cloudinit (the newer one isn't using that service at all.)
Any idea if it's possible to update the /usr/share/oem on the older VM to match the newer one? Or will this require a rebuild (which given the state of CoreOS, probably won't be possible right now, and may have to wait for official Flatcar images to be published.)
Not super important as the VM seems to work fine even with the failed unit, but it would be nice to have them both using the same services to better integrate with the Google Cloud platform.
The way the upgrade mechanism works (both how it worked in CoreOS and how it now works for Flatcar) the /usr/share/oem partition is untouched after initial installation. This means that whatever was shipped with CoreOS when you installed your instances is what you'll have in those partitions.
As Adrian mentioned in his comment, you likely installed those instances at different points in time and that means that the contents are different.
Currently, there's no way to automatically update that partition. In the Flatcar team, we are working towards a solution for that, but it's not ready yet. For the time being, one thing you can do is install another machine with the latest version and manually copy the contents of that partition. Ugly, I know.
Official Flatcar images for GCE are coming soon, but in the meantime, you can install the unofficial images as described here