How does Ratings and Review work?

I have been trying to understand ratings business (thanks in part to Mohnish Pabrai's CARE purchase). One of things I keep wondering is how do ratings and reviews work.

For example, lets say I want to do a 1 crore issue. By law, I need to get 2 ratings so I approach CARE and CRISIL to get a rating. The issue gets rated to AA. After sometime there is a material change in business so now CARE and CRISIL have to review the rating. But somehow I don't like CRISIL's way of doing things, can I then ask ICRA to rate me instead?