For quite a while now, Pokémon GO has an improved buddy Pokémon system whereby you collect hearts by interacting with your buddy to unlock various friendship levels and additional perks.
Say I have Eevee as my buddy Pokémon and collect 30 hearts with it. Then I make Suicune my buddy. When I switch back to Eevee, will its buddy level be back to zero or will the progress have been kept?