This is tricky but bear with me. Your Daimyo accepts Christianity as his own religion once he builds or owns a Church. In that case the clan is given the Christian sign in the info menu and your provinces will start to gradually convert and opposition is met until the surpass the 50% limit.

However even if you don't build any Church and don't turn Christian yourself, the Portuguese will start spread Christianity once you accept a trade deal with them. This however mean that although Christianity can cause unrest the Daimyo and the majority of the (buddhist) subjects are not in opposition and the effect is far smaller.

In order for a Christian to turn Buddhist again all is needed is to build a Buddhist temple. Once this is done Christian status is lost again. However if now the majority of the population is Christian expect unrest anew.

By the way the only way for a Buddhist daimyo to boost Buddhism is via the temples, which are expensive. The Christian Daimyo on the other hand apart from Churches and the Cathedral can use the cheap and handy Jesuit priests that turn japan Christian in no time.