It will stay as Own Tempo. Abilities only change if the evolved form has different abilities than the pre-evolved form. For example, Lillipup has the abilities Pickup and Vital Spirit, but Herdier has Intimidate and Sand Rush. So when it evolves, Lillipup's ability will change. But this doesn't happen for Slowpoke, because Slowbro (and Slowking) have the exact same abilities.