Basically all Muslims east, north and south of Saudi Arabia are all descendents of converts who were forced into converting to Islam. I could be wrong about a few; but Afghanistan, Iran, Pakistan, India, Bangladesh, and points east, including Malayasia and Indonesia, were all either Hindus, Buddhists, or animists of one sort or another. (A few Jains and other sorts here and there.)
If only it wasn't against Muslim law for them to convert back!
Not entirely accurate. Any 15th century lower caste or untouchable Hindu in India had very strong incentives to convert to Islam.
In Islam, at least in theory, there is no such thing as caste or even class. In traditional Hindu society, there is absolutely no way to rise above your "place" in life.