Long story short, I had the Orbital Terraforming galactic achievement, had an alien race surrender to me, and none of their worlds improved in environment. And the race was pretty far behind me in technology, I doubt that they'd managed to get terraforming, much less gotten around to fully upgrading any of their planets. Whereas when I had the neutral morality bonus of all worlds being fully-upgraded, it was a fairly regular occurrence to capture a world and have it immediately upgrade a few ranks.

It's possible that their worlds were just crappy and didn't have any tiles that could be upgraded, but I took over three different races with about fifteen worlds between them and saw no improvement of any of those planets, whereas I'm used to fairly noticeable upgrades like a class 10 turning into a 16.