They've done it after the tournament
this article says it was before the 2001 tournament :
In 2001, Wimbledon tore out all its courts and planted a new variety of groundcover. The new grass was 100% perennial rye; the old courts had been a mix of 70% rye and 30% creeping red fescue. The new lawn was more durable, and allowed Wimbledon's groundsmen to keep the soil underneath drier and firmer. A firmer surface causes the ball to bounce higher. A high bounce is anathema to the serve-and-volley player, who relies on approach shots skidding low through the court. What's more, rye, unlike fescue, grows in tufts that stand straight up; these tufts slow a tennis ball down as it lands.
Ivanisevic and Rafter were able to blast their way through the new grass because an exceptionally rainy two weeks had kept the courts soft. But the ground eventually dried, and baseliners have excelled since
Read more: http://www.time.com/time/magazine/ar...#ixzz2JIHqGygP
Also, this French journalist who has followed tennis for long says it was between 2000 and 2001 :
Entre les éditions de 2000 et 2001, une nouvelle variété de gazon a été semée, il est vrai, sur tous les courts de Wimbledon après des études menées par The Sports Turf Research Institute, organisme situé dans le Yorkshire.