Rav Eliashev paskened that if someone underwent the geirus process all the while believing that the world is over 6,000 years old, that the conversion was never valid and he remains a gentile.
If someone undergoes conversion intending to fully keep 612 mitzvos with all the chumros but there is one mitzvah he intends to not maintain, his so-called coversion was never valid and he remains a gentile.