This already happened in 1986.

"The Immigration Reform and Control Act legalized most undocumented immigrants who had arrived in the country prior to January 1, 1982. The act altered U.S. immigration law by making it illegal to knowingly hire illegal immigrants, and establishing financial and other penalties for companies that employed illegal immigrants."

"By splitting the H-2 visa category created by the Immigration and Nationality Act of 1952, the 1986 law created the H-2A visa and H-2B visa categories, for temporary agricultural and non-agricultural workers, respectively."

"Despite the passage of the act, the population of undocumented immigrants rose from 5 million in 1986 to 11.1 million in 2013."

https://en.wikipedia.org/wiki/Immigration_Reform_and_Control...