Ethereum will never be Bitcoin and vice versa. Bitcoin was the first cryptocurrency, it is still most popular and recognizable. Ethereum on the other side is widely used, so I see no reason they can switch places. Each one has its own place in the world and in the history.
Maybe it possible to happen to be next bitcoin because it has a lot of a potential like the higher value and and its popularity. But i think bitcoin is still be the number one because it is the most popular cryptocurrency until now.
Also, those who are saying that ETH will be the next BTC are all ignorant. How can you say that? Maybe a competitive market cap between them sounds real but replacing one with the other is quiet not possible. They have their own identity, something it does which the other can't. BTC is a decetntralized P2P while ETH has its own smart contracts.