"People change religion... How can one force them to readopt the religion of their ancestors?"

From the perspective of modern legal systems that protect freedom of religion, people are generally treated as having the right to adopt, change, or reject a religion.

Therefore, the fact that someone's ancestors followed a different religion does not ordinarily create a legal obligation for that person to adopt the ancestors' religion.

This is because legal identity is generally based on a person's present legal status and choices, not solely on ancestral affiliation.