When 2345 was installed this morning, all my network renaming rules stopped working.

This is in the release notes, and broke it coreos/bugs#1767

My link file is pretty simple:
[Match]
MacAddress=....

[Link]
Name=srv0

This worked in previous stables releases of coreos and flatcar.

My server was in-place upgraded to flatcar per your documentation.

Upon running
SYSTEMD_LOG_LEVEL=debug udevadm test-builtin net_setup_link /sys/class/net/ens192

It indicates the default naming scheme is v238 - this naming scheme IMPLICITLY has the NamePolicy "keep" behavior applied. The keep option isn't available until v240 - BEFORE v240 it is ALWAYS applied even if you try to clear it. So what happens is in the initrd, eth0 gets renamed to ens192, and then when non-initrd systemd picks it up, it won't rename something that the "userspace" has already renamed. Ever.

I had to add this
set linux_append="net.naming-scheme=latest"
to my /usr/share/oem/grub.cfg to fix it. After boot, the interfaces are renamed properly, and udevadm shows it's using v240.

If there's a more permanent fix to this, or this needs to be documented, or re-changed, or the coreos to flatcar upgrade process needs another step... I don't know. Please investigate.