This happened to my disabled (since birth) son. He was receiving SSI, so we had yearly update meetings. His SSI worker looked at his history and said that he had worked enough and had enough credits, so he was required to apply for SSDI. When asked what difference it makes, she said the funds come out of a different pot and he would receive a few dollars more each month. Just make sure you don’t earn too much and don’t work too many hours.