Hello Summoner,
Thank you for contacting player support. I do understand your concern. It does not guarantee you that when you transfer your account you'll still retain your current standing in the League.
After the transfer you will have to complete one placement game in the solo ranked queue. The results of that game combined with your previous rating will determine your League placement on the new server. Please note that because of the differences in the competitive environments between each platform you might end up in a different division or even different tier than before. Rest assured that your hidden matchmaking rating stays the same and you will continue to play with opponents of approximately your level of skill.