An unbreakable spiritual chain interlinks a soul, a soul-group, and a religion. A soul belonging to a particular soul-group has a particular spiritual requirement which only a particular religion is designed to satisfy. Therefore, the religion to which a soul needs to belong is a settled matter even before it actually incarnates. This preordained tie between a soul and its matching religion cannot be established on earth through any criterion other than the benchmark of birth. Birth and religion are inseparably intertwined, and any later conversion is invalid in Nature.
Conversion is a spiritual impossibility.
The validity of this truism will be easily seen with the help of a more mundane analogy. It is exactly the same thing with the physical genes inherited at birth from our parents. These genes are unalterable throughout life, no matter what one may do. I can legally change my surname from Khan to Boomlachor, and to the world I will be Mr. But that does absolutely nothing to change my genetic Khan lineage.