It's actually just a minor inconvenience because I can reboot and hold down Option to get back into High Sierra.
I'm mainly hoping to find out if there is a way to "update" this existing boot camp installation so that it can find the new macOS volume again. That way I won't want to try to reinstall windows again.
I'm sure it's just some partition/volume handle setting stored somewhere in the bootcamp system in Windows.
diskutil list
andgpt
info - https://apple.stackexchange.com/a/176557/5472 – bmike Oct 01 '17 at 20:33