I don't think they have broken ground on it yet, but this possibility has been floating around for a while as a path to upgrade/modernize the old stadium.
It is similar to what went on with Soldier Field, and now that has had the consequences of the Bears wanting something new anyway since Soldier Field is the smallest stadium in the league.
It is the end of an era as Arrowhead has long had a reputation of being a difficult venue for visiting teams, as well as not too many of the old legendary stadiums are around anymore.