Immediately, it sounded as though an abrupt change had occurred. In reality, it was something more gradual, more of an evolution.

When its doubly self-titled sophomore record first hit speakers last spring, Bon Iver began to feel less like it was merely the nom-de-stage of a solo singer-songwriter, a conventional guy-with-a-guitar, one Justin Vernon. The acoustic introspection and haunting isolation of its 2008 debut, For Emma, Foever Ago grew into something lush and expansive on Bon Iver, Bon Iver, with emotive playing and majestic arrangements. On the new album, Bon Iver began to feel like a band.

This was, after all, inevitable. As Vernon told WXPN’s David Dye when he was interviewed for World Café last autumn, the first record was borne out of a highly introspective time. “There was a little bit of tail-between-my-legs going on,” he acknowledges of writing it following the end of a romantic relationship and the breakup of his old band, DeYarmond Edison. This element – working on music alone and sad in a cabin in the Wisconsin winter – was possibly overly mythologized, but nonetheless, For Emma was crafted as a collection of very personal songs, and sounds like one.

As soon as Vernon began performing these songs under the guise of Bon Iver, they began to morph. Solo shows grew into two-piece performances with drummer Sean Carey, then further into quartet configuration with guitarist Michael Noyce and bassist Matthew McCaughey, all the way to the nine-piece ensemble that will play The Mann Center for the Performing Arts on Sunday.

“Everyone just kind of came into my life at different times,” Vernon told Dye on the Café. “And really, the first time we ever got together as a full nine-piece, a few guys had never met each other.”

Despite this, revamping the older material – like “Beach Baby” from the Blood Bank EP – for the new configuration of Bon Iver happened easily and organically. “I think that everyone was semi-familiar with the music,” Vernon continues. “But everyone that was handpicked was a good person and a strong musician and also a strong character. I think that, when we all got together, we just approached it and it just kind of happened. It was kind of stunning actually that we all just kind of had ideas, we all listened to each other’s ideas and it was naturally, it was easy to roll ‘em out.”

Another growth that Bon Iver experienced in its transition from solo project into band – aside from the sheer numbers of players brought on board – was the more refined, rounded sound of the music. On For Emma, Vernon’s voice and phrasing is pained, jagged and husky – on Bon Iver, Bon Iver, it’s practically angelic.

“Something that was underappreciated by me as a songwriter growing up is how you deliver a line,” Vernon told Dye. “Sometimes you have to have the right mind to deliver for it to stick in there. You know, Bob Dylan is great at that, with how good he can make a word sound and put it in a pocket. I think that it’s starting to develop in my brain a little bit.”

This carries to the instrumentation. Even though the band is big, and sounds big, there’s a great deal of sonic restraint in play. Warm guitar tones and solitary vocals open “Perth” before the drums and band start a minute into the song. “Calgary” simmers to a swell of synthesizer and choral vocals. The use of space is generous and judicious. Vernon famously contributed vocals to Kanye West’s My Beautiful Dark Twisted Fantasy, and that experience had him working with craftsmen of the opposite extreme. He described to Dye studio technicians spending eight hours to get a snare drum sound just right.

“When you’re working on a record like that, you can really get away with throwing a lot at it and then dealing with editing it,” Vernon said. “Kind of constructing and deconstructing and constructing. I think that those experiences in the last few years of recording really definitely informed me as I went forward.”

A final growth worth noting – Vernon’s transition from small one-man-band to indie royalty, an artist with a million-plus Facebook followers, 288,000 Twitter followers. The clout this guy has in music right now is substantial, and even though many still view him as the sole member of this epic ensemble, he shrugs it off.

“I think that you could decide to be a famous guy and to act a certain way,” Vernon mused. “Or you can just decide to be who you are, and I think that’s a lot more comfortable and sane and accurate for me.”

Bon Iver plays The Mann Center for the Performing Arts on Sunday, September 16 at 7:30 p.m., with Anais Mitchell opening. Tickets to the all-ages show range from $29.50 to $49.50 and are available here. Listen to Bon Iver’s World Cafe session and interview in its entirety here (via the WXPN media player).