Honestly, it isn't very clear. Different people have different versions.
Let's understand "Scheduled Caste" first.
Originally, it was a backward group that was exclusive to Hinduism and its caste system.
However, the Indian constitution stretched its meaning with a secular undertone. This was done to widen the net of our reservation system. So, now, Schedule Caste exists in all the various religions.
(You will find SC people that are Hindus, as well as Muslims, Christians and belong to other religions. Per our 2011 consensus, 16.6 percent of India's population belong to scheduled castes.)
Indian constitution defines Schedule Caste, irrespective of religion, as a class category that is, historically, at the disadvantageous position in the society both socially and economically.
So, technically, now, religion isn’t directly related to caste.
Meaning, if a Hindu converts to Christianity, that won’t affect his caste. Because per design, he was born in a caste. He carries that stamp by birth and not a choice. It cannot be changed. So, even with conversion in religion, he will continue enjoying the reservation quota, as well as other benefits, that his caste is given by the constitution.
(Courtesy: Indian Folk)
But aside from this basic explanation, I am sure there exist many technicalities to it that can refute this argument.
In addition, there could also be many other social problems. Foremost, changing religion in India is not as easy as our Constitution portrays; not at least in the current political climate. To that, when you change your religion, you're also going to have to change all your identifications, which is a BIG hassle even in this digital age. And then when you go to the respective offices to claim your reservation and other benefits, there's a good chance you would be harassed.
So, if you're really planning to change your religion, there's plenty more you must consider than just SC reservation quota.
But, in the end, to answer your question, converting to Christianity won't affect your reservation claim that you enjoy as a part of scheduled caste.