With a flourish of the governor’s pen, much of the bookstore fare at Bridgewater State College became collectors’ items. That’s because the merchandise — hats, teddy bears, and 15 varieties of T-shirts — will be outdated by the fall, when the school unveils a new logo for Bridgewater State University, its new name, as part of a law signed yesterday that will rebrand six state colleges as universities. Three other state colleges will retain their names, but become part of the new state university system. “It’s been decades in the making,’’ Frederick Clark, executive officer of the Council of Presidents of the Massachusetts State Colleges, said after Governor Deval Patrick signed the measure on Beacon Hill. “This gives our students and alumni competitive, even footing across the country and across the world. People will better understand what we do.’’ School officials said the change will enhance student and alumni résumés, help the schools compete for grants, and attract better faculty members.