POMS is the SSA manual and lays out specific instructions on everything they do. It tells them what they need from you, and how to update the system.
CDR is a Continuing Disability Review. SSA is required by law to do this at least every 7 years, which typically means more like 8 for a lot of people. They want to see if there's a change in your condition, which honestly isn't the worst thing which could happen given how an SSI check compares to a paycheck.
People do change their SSNs, by the way. If yours is stolen, for example, they can get you a new one.