In the window of Harmony Hut record store at 14th and F Streets this week, a slight, bespectacled man is attracting lunchtime crowds with his lively renditions of Scott Joplin rags.

Two years ago Charlie Howze, 48, former chief counsel to the House Subcommittee on Investigations and a Yale and University of Virginia Law School man, was busy practicing law. But in the last 18 months he's been phasing out his law practice and "setting out as a rookie at middle age, in a new career at the keyboard.

Howze has only been working intermittently since a long engagement at the Gaslight Club ended in May 1975. But enjoying himself.

He's not unusual in starting a new profession. Nearly one-third of all American workers may change their careers over a five-year period, according to the Labor Department's first major survey of career-switching based on the 1970 census.

Howze left law partly because he was tired of it - and he'd wanted to play music for a long time.

"I wasn't doing very well at it (law)," he says, sitting in an easy chair in his home in Northwest. "I'd gotten too old for private practice, and I hadn't done well in government lately (he had worked for the USIA and EEOC before entering private practice.)"

Now Howze is rapidly gaining self-confidence about his piano playing, and performs whereever jobs are available. He appeared at the Gaslight Club from June 1974 to May 1975. Since then, however, it has been hit-and-miss work - Fitzgerald's (now defunct), filling for John Eaton at the Carriage House and last night subbing for Jean Packard at the Washington Hilton.

The job Howze has at Harmony Hutends Saturday, and is set up to publicize a forthcoming film biography of Scott Joplin.

"It's been tough not having a steady income, but my wife, Dorothy, and I have a feminist household," says Howze, shushing away his two frisky beagles in the living room. "She has a good job as an equal opportunity specialist at the EEOC, so we haven't starved.

"My wife doesn't mind me changing careers. She's for it. She's just like me - she wishes I'd get more work."

As part of their liberated household, he helps clean house and shop for groceries. Howze's cooking is limited to fixing light breakfasts.

Starting a new career was scary at first, recalls the pianist-lawyer. Sometimes before job he'd drink loads of beer, just to relax.

Several pianists, including Mel Clement, Packard and Eaton, have encouraged Howze. For the last two years he's studied with Clement.

"I still need to get my technique up, and my strength and speed up and improve my knowledge of harmony," he comments. "If I've played a D-11 chord with a flatted fifth, I want to know what I'm playing. My friends like Mel and John tell me I play this stuff, but I don't know it."

Howze, who grew up in the Washington area, took his first piano lessons at age 6, but didn't take the instrument seriously until he was 12 or 13. He played sporadically until going off to the Army shortly after World War II. Later came to Yale and the University of Virginia Law School.

Howze says it's conceivable he could be tempted back to law, but doesn't think it likely.

Sitting at the spinet piano in his living room and careering the melody of "Spring is Here," he says: "I'm one of those day-to-day livers. I've never been a long-term planner. I'm a stubborn guy. I guess that's reflected in my musical tastes."