Imagine this. You've been waiting for the release of Sylveon in Pokémon GO for years. You have the perfect Eevee saved for the occasion. Shiny. Best Buddy. Lucky. Maybe even 100%. You do everything right. You earn 70 Buddy Hearts and you go to evolve your Sylveon… and instead, it ends up as an Umbreon or Espeon. That is the worry that was moving through the Pokémon GO community when Niantic failed to give information about a certain quirk of Eevee's evolution into Sylveon. Let's get into it.
Here's the issue. To evolve Eevee to Sylveon (after you have used the nickname method, of course), you make the Eevee of choice your Buddy and earn 7o Buddy Hearts with it. This seems fairly self-explanatory, but the issue is that Pokémon GO players didn't know how this would interact with the similar evolution method for Umbreon and Espeon.
To evolve Eevee to Umbreon or Espeon, you make your Eevee your Buddy and walk 10KM with it. Then, if you evolve it during the daytime in-game, you get Espeon. If you evolve it during the nighttime in-game, you get Umbreon. The problem here is that, while earning 7o Buddy Hearts with your Eevee, you are very likely to walk more than 10KM.
While we have yet to receive clarification from Niantic themselves, a Japanese Pokémon GO Twitter has now posted a screenshot showing an Eevee that has walked the 10KM while also earning the 70 Buddy hearts, answering the question that players had.
The answer is that there will be two evolution buttons. One for either Umbreon or Espeon, and one for Sylveon. You will be able to differentiate it by both the Pokémon's silhouettes as well as symbols on the button. Espeon will have a sun symbol for day, Umbreon will have a moon symbol for night, and Sylveon will have a Buddy symbol.
So there it is! A very straightforward and seemingly obvious answer, with the only real issue being that Niantic didn't inform Pokémon GO players of this themselves.