TOKYO—Nintendo Co. plans to release a new version of its Switch videogame console next year to maintain the sales momentum of the device, according to suppliers and others with direct knowledge of the plan.

Sales of the Switch, introduced in March 2017, are still solid but are no longer delivering the favorable surprises that marked the machine’s first year on the market. Nintendo shares, which rose sharply last year, have trailed the broader stock market this year. The move to update the Switch suggests the Kyoto, Japan,...