Not all muslims seem to adhere to their own rules but I have read that traditionally only Jews and Christians are allowed to convert. Being “people of the book” as they call it. All others can be dhimmis, slaves or corpses but never muslims.
Actually, “People of the Book” are allowed to keep their religion IF they agree to pay the Jizya and submit to Dhimmitude with it’s discriminatory restrictions. Non-PotB, like pagans, for instance, had to convert to Islam or die. Exceptions were made, like India, but probably because there were too many Hindus to kill.
If that’s the case, what about Malaysia, Indonesia, Pakistan, or Bangladesh? They were all originally Hindu.