The Seattle Post-Intelligencer reported the move is not a surprise because the 6-foot-10 center said as early as the 2005-06 season he planned to do it.
Last season, the eight-year veteran had per-game averages of 22.4 points and nearly seven rebounds in 60 contests.
Lewis is the Sonics' longest-tenured player and was their second-round pick in the 1998 NBA draft.
He signed a seven-year, $60 million contract during the 2003-04 season, for which he still is owed $21 million.
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