If a person traces his/her parents lineage and discovers that they indeed are Jewish many decades ago does that person have to go through a form of conversion, especially if they were raised as a Christians?
I am very happy that you are looking for your Jewish roots. There is a story about the grand son of Rav Yaakov Kaminetzky who was showing much respet to his grandfather. A non Jew who was sitting in front of them asked Reb Yaakov the reason for this great respect, since he was not shown any respect by his own grandchildren. Reb Yaakov answered that for those who believe that man comes from the monkey, then the earlier the generation the closer hie his to the monkey and does not deserve respect. But we no that G-d created man, and the earlier the generation the closer he is to the creation and to G-d, and deserves more respect. The answer to your question is that if you have a valid witness that your parents or grandparents were Jewish, then you need only the tvila of baalei tshuva (immersion required of a Jew returning from a foreign religion). If you have no witness that you have a Jewish lineage, then you need to go through a full proper conversion. You should show the evidence you have found of you Jewish lineage to a rabbi to determine if it constitutes valid testimony by Torah law. Rabbi Moshe Klein