Some artists undergo reinventions. What Nathaniel Rateliff accomplishes on his first LP with the Night Sweats — transforming himself from folk troubadour to funky soul bandleader — is a revelation. Rateliff comes on like Van Morrison fronting Booker T. & the MG’s, reviving the glory days of his label, Stax. The robust opener, “I Need Never Get Old,” sets the fever pitch with its chugging groove and blaring horns; the rowdy “S.O.B.” is just itching for a bar brawl. Even so, Rateliff hasn’t completely forgotten his folkie past: The wistful “Wasting Time” shows that he can still kill you softly.