I wish the game would somehow visually indicate which upgrades you've never bought before. I feel like it would make it easier to unlock 100% of the entries in the beecarbopedia.
I found out this information is availible in the encyclopedia, you just have to look at each card description to see what they turn into. Somehow I'm still missing one Nature card, though. Maybe it gets auto-built when you fail an event, or something? I googled it. Apparently I got good at the game too quickly. I needed to let pollution pile up until I get a specific event, then succeed at that event.
I'll just play a round and keep 20th century Industry around for a while.