One of the upgrades says it increases investor effectiveness by 2 percent and it costs 100,000 investors. If you buy this upgrade, does it only affect the investors you get from the next reset, or from all of them? Does it also affect the investors you already have?
It applies immediately. You'll see in the investors menu that the Bonus per Angel is increased by 2%. The upgrade is reset (just like all upgrades) when you reset the game to get more investors.
I am not sure what the percentage bonus is before you buy it, but I think it is 5%. So you'll get from 5% bonus to 7%. That means the minimal amount of investors to purchase this upgrade is 350.000. (350.000 * 5% = 1.750.000 % bonus, 250.000 * 7% = 1.750.000% bonus)