> To listen, go to bostonglobe.com/soundtrack. Outside, the season’s transformations are everywhere: lilacs blooming, baseball diamonds filling, schoolchildren shedding layers like sheep sheared of their wool. At Symphony Hall, the Boston Symphony Orchestra undergoes its own spring metamorphosis, putting aside large-scale orchestral pieces to make room for the more playful, more accessible repertoire of the Boston Pops. The change requires a major reworking of the hall, with the auditorium-style seating replaced by small tables and folding chairs. It also means a shift in mind-set for the musicians, many of whom play in the Pops and the BSO. (Over the summer, both ensembles perform at Tanglewood, in the Berkshires.) On May 8, the day the Pops opened its spring season with country legend Vince Gill, I sat in on the afternoon rehearsal. Unplanned harmonies and duets filled the corridors and the stage as musicians warmed up on their own. New BSO member Michael Winter sat on a black travel case beneath the concert hall, test-driving his horn over the hiss of nearby pipes. A little after 1 p.m., with Gill and his band ready to go, conductor Keith Lockhart led the group though “When Love Finds You,” from Gill’s hit 1994 album. With Gill in shorts and sneakers, it looked like a rehearsal, all right. But it sure didn’t sound like one.