CARACAS/HOUSTON (Reuters) – A subtle change in office attire may be the most telling symbol of a quiet revolution taking place inside Venezuela’s troubled economic engine, giant oil firm PDVSA. For years, PDVSA employees were encouraged to wear red shirts in support of late President Hugo Chavez’s socialist movement. Rafael Ramirez, the former oil czar famously vowed the state-owned firm would be “redder than red” and sent workers to state rallies.