How did the course of Reconstruction change when the Radical Republicans took control of Congress?

They now had the power to override Johnson's vetoes and pass the Civil Rights Act and the bill to extend the Freedmen's Bureau

Explanation:

While Lincoln took a moderate approach to Reconstruction, Congress sought to impose ... Radical Republicans believed that Lincoln's plan for Reconstruction was not harsh enough.

Radical Republicans won over two-thirds of the seats in the House of Representatives and the Senate. They now had the power to override Johnson's vetoes and pass the Civil Rights Act and the bill to extend the Freedmen's Bureau, and they did so immediately. Congress had now taken charge of the South's reconstruction.

Republican governments collapsed, thereby ending Reconstruction. Over the next three decades, the civil rights that blacks had been promised during Reconstruction crumbled under white rule in the south.