Answer by taking a quote from the following article:

Lester Willis Young (August 27, 1909 - March 15, 1959), nicknamed "Pres" or "Prez", was an American jazz tenor saxophonist and occasional clarinetist. Coming to prominence while a member of Count Basie's orchestra, Young was one of the most influential players on his instrument. In contrast to many of his hard-driving peers, Young played with a relaxed, cool tone and used sophisticated harmonies, using what one critic called "a free-floating style, wheeling and diving like a gull, banking with low, funky riffs that pleased dancers and listeners alike". Known for his hip, introverted style, he invented or popularized much of the hipster jargon which came to be associated with the music.

Young left the Basie band in late 1940. He is rumored to have refused to play with the band on Friday, December 13 of that year for superstitious reasons spurring his dismissal, although Young and drummer Jo Jones would later state that his departure had been in the works for months. He subsequently led a number of small groups that often included his brother, drummer Lee Young, for the next couple of years; live and broadcast recordings from this period exist.  During this period Young accompanied the singer Billie Holiday in a couple of studio sessions (during 1937 - 1941 period) and also made a small set of recordings with Nat "King" Cole (their first of several collaborations) in June 1942. His studio recordings are relatively sparse during the 1942 to 1943 period, largely due to the recording ban by the American Federation of Musicians. Small record labels not bound by union contracts continued to record and he recorded some sessions for Harry Lim's Keynote label in 1943.  In December 1943 Young returned to the Basie fold for a 10-month stint, cut short by his being drafted into the army during World War II (see below). Recordings made during this and subsequent periods suggest Young was beginning to make much greater use of a plastic reed, which tended to give his playing a somewhat heavier, breathier tone (although still quite smooth compared to that of many other players). While he never abandoned the cane reed, he used the plastic reed a significant share of the time from 1943 until the end of his life. Another cause for the thickening of his tone around this time was a change in saxophone mouthpiece from a metal Otto Link to an ebonite Brilhart. In August 1944 Young appeared alongside drummer Jo Jones, trumpeter Harry "Sweets" Edison, and fellow tenor saxophonist Illinois Jacquet in Gjon Mili's short film Jammin' the Blues.

Why did he change to a plastic reed?
which tended to give his playing a somewhat heavier, breathier tone