This only happens if KG decides to retire. Pierce will be on this team until then.
Now, depending on how Pierce looks compared to Green for the remainder of this season, I wouldn't rule out switching their roles.
Maybe during the off-season, Doc has a talk with Pierce, and sees if he would accept a move to the bench. Pierce would still finish games, but with reduced minutes against lesser players, it might be to his and the team's benefit. This is stictly hypothetical though.