Answer by taking a quote from the following article:

Muddy Waters' birthplace and date are not conclusively known. He stated that he was born in Rolling Fork, Mississippi, in 1915, but it is believed to be more likely that he was born in Jug's Corner, in neighboring Issaquena County, in 1913. Recent research has uncovered documentation showing that in the 1930s and 1940s, before his rise to fame, the year of his birth was reported as 1913 on his marriage license, recording notes, and musicians' union card. A 1955 interview in the Chicago Defender is the earliest in which he stated 1915 as the year of his birth, and he continued to say this in interviews from that point onward.

Initially, the Chess brothers would not allow Muddy Waters to use his working band in the recording studio; instead, he was provided with a backing bass by Ernest "Big" Crawford or by musicians assembled specifically for the recording session, including "Baby Face" Leroy Foster and Johnny Jones. Gradually, Chess relented, and by September 1953 he was recording with one of the most acclaimed blues groups in history: Little Walter Jacobs on harmonica, Jimmy Rogers on guitar, Elga Edmonds (also known as Elgin Evans) on drums, and Otis Spann on piano. The band recorded a series of blues classics during the early 1950s, some with the help of the bassist and songwriter Willie Dixon, including "Hoochie Coochie Man", "I Just Want to Make Love to You", and "I'm Ready".  Along with his former harmonica player Little Walter Jacobs and recent southern transplant Howlin' Wolf, Muddy Waters reigned over the early 1950s Chicago blues scene, his band becoming a proving ground for some of the city's best blues talent. Little Walter continued a collaborative relationship long after he left Muddy Waters's band in 1952, appearing on most of the band's classic recordings in the 1950s. Muddy Waters developed a long-running, generally good-natured rivalry with Wolf. The success of his ensemble paved the way for others in his group to make their own solo careers. In 1952, Little Walter left when his single "Juke" became a hit, and in 1955, Rogers quit to work exclusively with his own band, which had been a sideline until that time.  During the mid-1950s, Muddy Waters' singles were frequently on Billboard magazine's various Rhythm & Blues charts including "Sugar Sweet" in 1955 and "Trouble No More", "Forty Days and Forty Nights", and "Don't Go No Farther" in 1956. 1956 also saw the release of one of his best-known numbers, "Got My Mojo Working", although it did not appear on the charts. However, by the late 1950s, his singles success had come to an end, with only "Close to You" reaching the chart in 1958. Also in 1958, Chess released Muddy Waters' first album, The Best of Muddy Waters, which collected twelve of his singles up to 1956.

What changed their mind?