Question

how are corporate bond ratings determined? and how and why do these ratings change once they are arrived at?

Answer 1

Bond ratings are expressed as letters ranging from "AAA,"(highest) to "C" or "D"(lowest).
ratings are developed based on specific internal and external influences such as initial yield of the bond and financial health analysis that include the amount of income a borrower has to pay off the bond and the currency in which the bond is issued and if any security used as a basis of repayment for the bond is also considered. firms, which are safer investments, have a high rating, while risky companies have a low rating

Bonds are rated at the time they are issued, and both bonds and their issuers are periodically analyzed by bond rating agencies and see if there is a change in rating of not. Economic conditions and issuer's overall ability to meet its financial obligations, plays important role in change in bond rating.