Can Generation 2 Pokémon (e.g. Pichu) hatch from an egg I got before the update that added them?
Unfortunately, all the eggs you got before that update don't contain any Pokémon from the second generation.
The content of an egg is determined at the moment you get the egg. So if your egg contained, for instance, a Charmander, it's because the egg has been determined to contain a Charmander at the exact moment you obtained it at the Pokéstop. During the update, the old eggs content has not been calculated once again, and they still contain what you were supposed to get at that point. So you can't get any second generation Pokémon from those. You will have to hatch the new eggs you obtained after the update.
This information seems confirmed on different pages and topics, like this one:
- Baby Pokémon Only Hatch from “New” Eggs
If you have Eggs in your bag obtained prior to the Gen 2 update in Pokémon Go , they will not yield the new Pokémon that come with the update.
Unfortunately, trainers will have to hatch these eggs first to make room for the Gen 2 Eggs they pick up at Pokéstops. This is because the identity of the Pokémon inside an Egg is determined the moment you get it from a Pokéstop so it only makes sense that “older” eggs won’t have newer Pokémon.
I don't think so, because eggs are already predetermined. This is so, because no matter at what level you are right now, the Pokémon that hatches is limited by the level you were when you got that egg.
When you received those eggs 2nd generation Pokemon were not present in the game yet. So it's not possible, eggs obtained before the update only contain 1st generation Pokemon.
See this question.
Also, from this Reddit:
Are hatched Pokemon determined when egg is collected or hatched?
As I recall someone who was 'sniffing' the PokeGo code being sent/received looking for 'missing' Pokemon because the number listed did not match the number possessed and determined that the number listed at the top of your bag (i.e. 150/250) included eggs.
He was also able to pull type and CP of his 'missing' Pokemon, which would indicate what eggs hatch into is determined when they are generated.