From Bulbanews, your community Pokémon newspaper.
Eevee Egg distance decreased
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Server-side changes have been applied to Pokémon GO which make common Pokémon appear less frequently.
Areas in which Rattata, Pidgey, and Zubat were commonly found will now also yield other Pokémon.
Additionally, Rattata and Pidgey can now no longer hatch from newly obtained Eggs. 5 km Eggs can now hatch into Eevee; previously it could only be hatched from 10 km Eggs. The distance needed to walk to obtain Candy with Rattata, Pidgey, or Eevee as the Buddy Pokémon (which corresponds to Egg distance) remains unchanged.
These changes are now live, and do not require an update.