Brandon Harris traded in a headset for a helmet.

LSU traded a game-managing sophomore for a gun-slinging freshman.

Inserted for a struggling Anthony Jennings, Harris completed his first five passes, threw for 178 yards and sparked the 17th-ranked Tigers to a 63-7 win over New Mexico State on Saturday night in what may have been a rousing opening act for a new era at quarterback for LSU.

Harris led the Tigers to touchdowns on his first seven drives, taking over on the game’s eighth series to loud applause from a two-thirds-full Tiger Stadium.

Serenaded with chants, Harris slipped off the headset — connected to assistant coaches in the press box — and grabbed his helmet.

He pulled the Tigers (4-1) out of a sluggish offensive hole, using rifling passes and quick feet to leave LSU fans pouring into the night in celebration of, yes, an expected victory and a possible change of guard at the most critical position on the field.

“When that guy comes into the game, he just makes plays for us,” receiver Travin Dural said. “That gives us a boost of energy. We get more tuned in the game. We’re more confident once we start making those plays.”

The now seventh-month-long quarterback battle between Harris and Jennings took its most pivotal turn Saturday — and just before the Tigers march into back-to-back SEC road games at No. 5 Auburn and Florida.

Coach Les Miles declined to name a starter for next week’s clash on the Plains during his postgame news conference, but Harris stole any thunder Jennings ever had.

Jennings threw two interceptions, lost a fumble, was sacked and completed just 2 of 5 passes in the first eight series — his only playing time. The Jennings-led offense scored just one touchdown, and boos rained down from the crowd.

Chants of “We want Harris!” thundered from a rowdy student section, and even former LSU players took to Twitter to voice their opinions: “We Want Harris,” tweeted Jeremy Hill, now a running back for the Cincinnati Bengals who served as a guest captain for Saturday’s game.

He later deleted the tweet, but he got what he wanted. And so did the fans.

Harris, a highly rated dual-threat quarterback from Bossier City, torched a struggling Aggies (2-3) secondary. He marched LSU on touchdown drives of 80, 62, 70, 3, 75, 74 and 65 yards.

Harris capped the final drive in booming fashion. He lowered his shoulder on a busted play, running in from the 5-yard line as the less than 30,000 left in Tiger Stadium thundered to life.

“(He) made plays. Very, very positive,” Miles said. “Not looking past this one. Seven straight scoring drives. Operated the offense successfully.”

The coach said he plans to speak with Jennings about the boos but said he hopes the quarterback hasn’t lost confidence.

Said Dural of Jennings: “He looked pretty down.”

Harris chided fans for the boos.

“It’s not something I’m for at all,” said Harris, allowed to speak to reporters for the first time this season. “I think we need to rally behind Anthony.”

Harris has thrown 30 passes this season, completing 22 for 394 yards, and has six touchdowns passes. Going back to the 34-29 loss against Mississippi State, Harris has led LSU to touchdowns on nine of his past 10 series.

Those numbers and his snazzy play overshadow anything from Jennings, a sophomore from Georgia. Jennings never even got time late in this one; third-string quarterback Brad Kragthorpe took over for Harris.

While it’s unclear who will start at Auburn (4-0, 1-0), Harris said he’s ready to face the reigning conference champions.

“I never get nervous in situations like that,” Harris said. “My body language may say I’m nervous, but I just never get nervous.”

Meanwhile, LSU’s defense forced four turnovers, and backup middle linebacker Kendell Beckwith returned a tipped pass for a touchdown in the first quarter.

Freshman running back Leonard Fournette rolled up a season-high 122 yards on 18 carries, scoring two touchdowns, and the Tigers ran for 363 yards.

Malachi Dupre, Trey Quinn and Travin Dural each had three catches, and the Tigers tied for the most points in Miles’ 10 seasons at the school.

The real show, though, was Harris. He hit Dupre (27 yards), Dural (10) and fullback Melvin Jones (3) on scoring passes.

“He’s been doing it since high school,” Fournette said of Harris. “Brought it to the collegiate level.”

Harris ran for 36 yards and completed 11 of 14 passes, showing off the strong, accurate arm that had him ranked the No. 3 dual-threat QB in the nation.

A Parkway High product, Harris enrolled early in January and immediately began competing with Jennings for the No. 1 job. He wowed spectators at the Tigers spring game but lost the starting gig during a back-and-forth battle in August.

Harris got three snaps in the first half in the season-opening win over Wisconsin and then saw nothing but mop-up duty over the next two games — blowout wins over Sam Houston State and Louisiana-Monroe.

A turning point came last week in the home loss to Mississippi State. He entered for an injured Jennings with 3:43 left and LSU down 34-16. He threw for 140 yards and two touchdowns in the final four minutes, nearly helping the Tigers pull of an improbable comeback.

On Saturday, he continued his successful engineering of LSU’s offense, marching the Tigers down the field — something Jennings couldn’t do.

Said Harris: “I’ll probably start looking at Auburn around midnight or 1 (a.m.).”

Follow Ross Dellenger on Twitter @DellengerAdv. For more coverage of LSU football, read our Tiger Tracks blog at blogs.theadvocate.com/tigertracks