Michelman, who begins in January, is currently a senior partner and West Coast regional president at PR agency Fleishman-Hillard.
'This move is about personal growth,' said Michelman, who has worked at F-H since 1999, heading the West Coast operations and acting as CEO of the firm's global technology arm.
'Visa is one of the world's great brands. And this is an opportunity to make an impact at a new level - to set a communications agenda to grow the business,' he adds.
Carmichael, who joined Visa a year ago, is leaving to spend time with her husband and daughter in Virginia.
'I want to get back and reintroduce myself to my family,' said Carmichael, who presided over a recent pitch in the US that moved Visa's US PR from Ketchum to Golin/Harris International.
The five-month-long review ended in November and saw Ketchum lose the £1.9m brief after six years (PRWeek, 15 November).
GCI Group - Visa International's retained agency - was unaffected by the review.
Until Michelman arrives, senior vice-president of corporate relations Albert Coscia will oversee Visa's PR team.
At Fleishman, Richard Kline, a senior partner and regional president who oversees Canada and Latin America, will become interim West Coast regional president in place of Michelman.
Kline joined F-H five years ago after a period as president of the then Shandwick USA.
He also previously acted as chief operating officer of Golin/Harris, which merged in 1988 with the agency he co-founded, Los Angeles-based Berkhemer & Kline.
Visa USA's comms push over the year ahead will focus on increasing it's share of the payment market in a bid to turn people away from cash and cheques.
According to Golin/Harris president Fred Cook, the agency will be 'further establishing Visa USA as a thought leader, developing its brand and addressing payment-industry issues'.