1. The different forms of marriage in the different religions are recognized in the Shar i 'ah. If both, husband and wife, accept Islam, there is no need to repeat their nikah. The nikah that they had performed as kuffar will still be valid.
2. If the husband or the wife accepts Islam and the other partner does not accept, their nikah will be annulled. It will not be permissible for them to live as husband and wife.
3. If the wife accepts Islam and not the husband, then as long as the wife does not complete three hayd periods, it will not be permissible for her to marry another person. ( By Moulana Ashraf Ali Thanvi RH).