High Sierra installation completed without error - but I'm still on Mavericks!

Greetings! I am running Mavericks on a Retina, 15-inch, Mid 2014 MacBook Pro and need to upgrade to High Sierra.

I ran the High Sierra upgrade. It apparently went fine, my machine rebooted - but I'm still now in Mavericks (everything looks the same, and the "About This Mac" menu item reports Mavericks). I rebooted a couple of times since, no change.

When I open App Store and go to the "Updates" tab, the button for High Sierra has changed to "Installed" and is now greyed out! (I tried to click it anyway, but no go.)

Interestingly, if I go to the "Purchased" tab, I can see "OS X Mavericks" and "OS X Mountain Lion" but not "OS X High Sierra".

I tried using MacOS Recovery: "Option-Command-R: Upgrade to the latest macOS that is compatible with your Mac" - but this just tries to reinstall Mavericks.

Any solutions to this problem or suggestions as to how to diagnose it would be appreciated! Thanks in advance.

(For the hardcore, I do development on this machine, and some subtle things have changed after the install - something has happened, just not an upgrade to High Sierra. For example, I had clean installations of Python 3.5 and 3.6 and they were just gone, so suddenly all my unit tests broke! Luckily, I do all development from inside a virtualenv so I just had to reinstall two clean versions, it took almost no time at all, and I have complete backups, but it makes me nervous, and I worry that the machine has been destabilized somehow...)