What is the ruling on the prayer of a man who intended to combine the 'Isha and Tarawih prayers in one mosque, then prayed 'Isha and left to pray Tarawih in another mosque behind a different imam?

There is no harm in leaving the Tarawih prayer behind a specific Imam and going to another Imam whose voice is better, or who offers a lesson, even if the worshiper intended to perform it in the first mosque. However, if leaving the prayer behind the first Imam would lead to harm, such as thinking ill of him or the Imam, or grieving him, or alienating people from him, then it is preferable to pray behind him, because averting harm takes precedence over acquiring benefits.