Here what’s happen to me: I got 8 10KM eggs incubating right before the Halloween Updates released. I was not able to hatch a Lapras, which made me a bit depressed. However, the next 10KM egg, getting after the Halloween updates (acquired by a pokestop near a river) was turned out to be my only Lapras, which completed my Pokedex.(not including the Regional specific pokemon) I got a Pokebuddy, who shares the same pain as me, was even more luckier. After missing his first 650 eggs for a Lapras, he got two Lapras during the Halloween events. I am not saying you are guaranteed getting a Lapras during that event, but a much higher chance for you if you were not lucky enough before.
Niantic is trying to have minor adjustment of the hatch rate of Pokemon from eggs for every update, i.e. the Eevee adjustment from 10KM to 5KM. However, it is only a ex-post concept. The change of hatch rate of pokemon will only happen after the update is completed. So, trainers, time to hatch all your current eggs and prepare spots for getting new Eggs!
Here are two things to be reminded of: 1. Eggs from pokestops at Starbucks and Sprints might give you a higher chance to hatch a Gen II Pokemon. 2. Hatch rate for Snorlax from a 10KM egg is expected to be increase during the Christmas Event.
That’s all for today! Thank you for reading!