Micklethwaite, who was head of brand strategy at the supermarket chain, has returned to Arla Foods, which owns the Lurpak and Anchor brands, having left the company to join Morrisons in June.
His previous role at Arla was vice-president of marketing, but Micklethwaite has rejoined in the more senior position of vice-president for strategy and portfolio.
Sonia Whiteley-Guest is replacing Micklethwaite at Morrisons with the job title of brand development director. Whiteley-Guest joins from Everything Everywhere, where she was the retail director for Orange. At Morrisons she will report to own-brand director Belinda Youngs.
Whiteley-Guest began her career at Asda, where she spent eight years and rose to become head of marketing for the George clothing range. She also managed the Extra Special and Good For You own-labels during her time at the supermarket.
The job changes come at a critical time for Morrisons. It has launched a major overhaul of its own-brand offering after a year-long review.
At Morrisons, own-brand products account for 43% of sales, while at its main rivals the figure is at least 50%.
The supermarket started the overhaul process by rolling out an M Kitchen brand last month, developed with executive chef Neil Nugent and including a Bistro line.
Morrisons also opened an unbranded pop-up restaurant in central London last month to promote its ready meals range.
The restaurant opened on Great Marlborough Street and served dishes made by well-known chefs, including Aldo Zilli. The grocer also ran a TV campaign to push the range.
Morrisons' value range will be rebranded as M Savers, with a campaign scheduled for the New Year; its premium range, The Best, originally a Safeway brand, is expected to be ditched.