ONE OF THE TOUGHEST moves a rock a roll band can make is a style change. Since the music industry's profits now register in the billions, it seems that prefab formulas are the easiest way to the consumer's heart - a heart generally represented by a record shelf.

Truly talented artists, though, usually find a way to expand musically while continuing to maintain their popularity. Such efforts are often called "transition albums" - records which showcase a new direction or, at least , break some new ground. Some transition albums become a performer's masterpiece - Elton John's Goodbye Yellow Brick Road (2MCA-10003) for example, (Elton's latest, Blue Moves (2MCA-11004), was a self-admitted second transition album but was generally considered less than classic). Others, including Joni Mitchell's The Hissing of Summer Lawns (Asylum 1051), win few converts and, to make matters worse, fail to placate old fans. There is a real risk involved.

Two bands that have taken that risk and are better off for it are the Doobie Brothers and Hall and Oates. Both acts' previous releases could be termed transitions and both of their new ones prove that their latest styles are artistically advanced while remaining commercially viable.

The Doobie Brothers' Livin on the Fault Line (Warner Bros. BSK 3045) picks up where Takin' It to the Streets (Warner Bros. 2899) left off. The Doobies have now officials left their powerhouse boogie days behind and planted their feet firmly on new ground.

Formerly, the band built its reputation with crashing chords and thumping bass lines. On Takin' It to the Streets, the Brothers and producer Ted Templeman revamped their collective sound to accomodate Mike McDonald's unique vocal phrasings but still flashed signs of the old juggernaut during tunes like "Wheels of Fortune" and "Turn It Loose." With Livin' on the Fault Line, the Doobies maintain their inbred tightness and rock roots but have a musical flexibility that enables them to weave in an out of a wide variety of configurations.

"You're Made that Way" and the title cut are jazz-tinged, with subtle percussion riffs by Keith Knudsen and John Hartman that alternately use samba, bossa nova, swing, and basic four-four funk. "You Belong to Me" is a layered ballad with McDonald's voice soaring like a husky Steve Winwood while even the two tunes stamped from the "hit single" assembly line - "Little Darling (I Need You)" and "Echoes of Love" - offer far more harmonically than your average three-for-a-quarter jukebox fare.

"Chinatown" is the album's focal point, a culmination of all the individual elements fusing into a seamless but varied piece. The rhythms are supple and the guitar arrangement goes deeper than did a similar attempt a few albums back on "I Cheat the Hangman". As a centerpiece. "Chinatown" proves that the Doobie Brothers have taken the best of both their worlds.

It could be argued that Hall an Oates's style never changed so much as it became belatedly popular. After all. "She 's Gone" was a hit years after its initial release and that song put the duo on top and allowed their natural technique to earn mass acceptance.

That's basically true, but the Hall and Oates of Abandoned Luncheonette (Atlantic 7269) are not the Hall and Oates of Beauty on a Back Street (RCA AFL 1-2300).

Though Hall and Oates are a more derivative group than the Doobie Brothers, they too have forged a distinct style after releasing a transitional record. That transition. Bigger Than Both of Us (RCA-APL 1-1467) promised more than the white soul of sophisticated disco labels that the duo was saddled with and now Beauty on a Back Street consistently delivers more.

Don't Change and Bigger Than Both of Us utilized Chris Bond's slick production to exude the Philadelphia sound that Hall and Oates absorbed during their years in that area and both compositions are more than visceral. The vocals are urgent and plaintive and the music crackles with a suppressed electricity. (It is interesting to note that the song "Bigger Than Both of Us" appears here and not on the album of that title. There is no song "Beauty on a Back Street" - it is a lyric in the David Bowieesque "You Must Be Good for Something." Strange folk, these rock and rollers).

"Love Hurts (Love Heals)" is a perfectly realized crossover melody, far superior to "Do What You Want, Be What You Are," its predecessor on the last album, and the doo-wop of "Why Do Lovers Break Each Others' Hearts" makes the boys guides on a playful but effective nostalgia trip.

Both Livin' on the Fault Line and Beauty on a Back Street are the results of years of trying to find just the right combinations. Both albums show creative growth, are commercially acceptable, and sound even better on successive listenings than they do initially. It demonstrates that sometimes you can have your fans and treat them too.