Don´t evolve Espeon/Umbreon during this event...
...unless you walk 10 km. I thought it was 2 candies but my planned Umbreon turned out to be a decent Jolteon.
The only other explanation would be that you - despite what´s being stated here before - can´t switch buddy after 2 candies and then put it back when you´re evolving.
Answers
Switching is fine as long as it is your buddy again wen you evolve it. I've done it a few times. I reached 10km and 2 candies while walking at night (wanted an Espeon), and switched to another pokemon to keep walking that night. The next time I reached a full candy distance during the day, I re-buddied the Eevee and evolved it to Espeon.
The only conclusion is that obviously it still needs 10km walked, not just far enough to get 2 candies, so it is better to walk with other pokemon during this event.
Same here. Evolved to get a Espeon and got Jolteon. I thought I should have restart the app because I read something like that sometime ago. But nope, the problem seems to be the 10km needed.
I 'lost' 4 Chansey candies because I walked another Eevee. Fortunately I didn't evolve it to get another dissapointment :/