Not everything has happened in the same way. Some future King were converted out of necessity. An example is Mieszko I, he leave paganism for some reasons:
1- Christianity, in front of his subjects, raised him as one who acts in the name of God.
2- The conversion allowed to bring priests, experts in the organization of the State and the Church.
3- The elimination of the pagan beliefs allowed to weaken the tribal structures in favor of national unity.
4- Being close to the Holy Roman Empire, it helped the relationship between Mieszko I and Otto I.
5- Finally, the most important reason. Being in the period of the Crusades against the infidels and pagans, Mieszko I realized that
Poland could not be pagan anymore, as his territory would be threatened by military expeditions.
I think there are more cases like this, but the important thing is to understand the period. In all the pagan territories (Slavic, Baltic and Scandinavian) people obviously don't abandoned the old believes without resistance. To be honest, I think, however, there have been persecutions of the pagans who don't accept the christian faith, it is impossible to think otherwise. I have mentioned only the case that I know, more than that I don't know what to write. We should do a search on people who have converted or have introduced Christianity in the Slavic countries. When I have free time I will, that is, after the exam session will end at the university.