When Mark Astrinos is asked what he does, the response is typically muted. “I’ll just say, ‘I’m a CPA,’ and the conversation will end,” said the certified public accountant and financial planner with Libra Wealth LLC in San Francisco.

Not lately, though. Thanks to the sweeping tax-overhaul bill passed by Congress and signed this month by President Donald Trump, people now “light up and they’re so intrigued and they want to know how they’ll be affected,” he said.

In...