BALTIMORE — The key pieces of the Texas Rangers’ front office have remained largely unchanged the past several years, but that could be changing.A source confirmed an mlb.com report that San Diego has been granted permission to interview assistant general manager A.J. Preller for its GM vacancy after Josh Byrnes was fired last week.Preller has been a mainstay alongside GM Jon Daniels in the Rangers’ front office, along with fellow assistant GM Thad Levine. Levine was a candidate for the Astros‘ GM job after the 2011 season before removing his name from consideration.Preller was hired by the Rangers in 2004, one year before Daniels was hired to replace John Hart. Preller was promoted to assistant GM in November, and he oversees the Rangers’ player development and scouting departments. Levine was one of Daniels’ original hires.