When a museum paying tribute to China’s economic reforms opened here in December, visitors were welcomed by a panoramic sculpture depicting a local visit by Deng Xiaoping, hailed in Communist Party historyfor launching the country’s rise to prosperity.

Then in early June, the museum was closed for what it called “upgrading.” When it reopened in August, the sculpture was gone, replaced by video screens showing local development and a beige wall adorned with a quote from President Xi Jinping.

China’s Communist Party is celebrating the 40th anniversary of policy overhauls, widely credited to Deng, that transformed an impoverished country into the world’s second-largest economy. Yet much of the state-backed fanfare has focused instead on Mr. Xi, playing up his economic credentials while diluting Deng’s prominence in party lore.

“This is mythmaking in action,” said Julian Gewirtz, a historian at Harvard University who has studied Chinese economic reforms. In shaping narratives about China’s future, Mr. Xi is “claiming greater authorship over China’s past.”