Just a few weeks ago, C.J. Beathard was in learning mode, asking questions of veteran Brian Hoyer about the 49ers’ dense playbook and expansive play calls. Now Beathard suddenly is in the role of the old-hand, dispensing the same sort of tips and advice Hoyer gave him to newcomer Jimmy Garoppolo. The trade that sent Garoppolo to San Francisco on Tuesday turned the 49ers quarterback room upside down. Hoyer was cut and signed by one of his former teams, the New England Patriots. Garoppolo is now viewed as the 49ers quarterback of the future. And Beathard? He takes over the placeholder role that Hoyer was in to start the season. The rookie will start Sunday’s game against the Cardinals and likely the following contest against the New York Giants. After that? “Just stay positive and keep working,” Beathard said. “Just keep doing what I do, working hard and doing my best.” Beathard didn’t fare well in the 2 1/2 games he’s played. He’s completed a little more than half his passes and has thrown two touchdowns and three interceptions, one of which was returned for a score. But he’s also dealt with dismal conditions, including a steady rain in Philadelphia, the disappearance of his offensive line – it was missing three members by the end of Sunday’s game – and an injury to Pierre Garcon that kept the 49ers’ top receiver out for much of the second half of the Eagles game. Despite those obstacles, Beathard has exhibited toughness and resolve and has shown the one thing Kyle Shanahan holds most dear when it comes to quarterbacks: a willingness to remain in the pocket while keeping his eyes trained downfield.