Some dissatisfied lawyers wallow in their unhappiness, frozen in place by a hefty paycheck or fear of change. When practicing law doesn't turn out to be what they expected, they wrestle with their disappointment but discontentedly soldier on. Not David Watson. Earlier this year, after more than a decade practicing law, the Winchester resident and avid cyclist became the new executive director of the Massachusetts Bicycle Coalition, or MassBike, a nonprofit statewide advocacy group based at 20 Park Plaza. The move marked a dramatic career shift for Watson, who turned 40 in February, and it came with considerable financial sacrifice; his MassBike salary is roughly 60 percent less than he had been making as the in-house counsel for a Framingham high-tech company.