Ian, that is not expected at all, especially not booting with the old kernel.
Since you still have the old contents, can you paste the contents of /boot/extlinux/extlinux.conf?
BTW, which BananaPi do you use? I've updated all my BPi-M1 without issues, but it is a rule for me to update in this order:
1) yum and rpm
2) all but kernel
3) kernel
To maybe that is why it didn't happen to me.

Thanks.
Pablo.

Ran a 'yum update' on my Banana Pi firewall this morning and ended up
with an unbootable system.  (The previous kernel wouldn't boot either.)

I ended up copying my entire SD card, installing the latest image,
growing the / partition, and copying the old root filesystem over.
Ultimately, I was able to get everything up and running.

This system was originally installed with
CentOS-Userland-7-armv7hl-Minimal-1611-BananaPi.img.xz.

I'm mainly wondering if this was expected.  If so, did I miss the
warnings?  If not, I do still have the dump of the unbootable post-
upgrade SD card sitting around, if anyone who understands the boot
process on these things wants to investigate.

Either way, thanks for all the work that you all do!