"The two new models are the first to reflect Hyundai's more moderated version of its design language, and are key to energizing its growth," said Sang Alexander Koo, a professor and former designer with Hyundai's affiliate Kia Motors.
"While the new Sonata will help embrace conservative consumers, it will not have as much impact as the current model did... The new Sonata could draw as much a divisive view as the current model. Some will say the change is weak and others may like it because it's more moderate," Koo said.
(Read more: Automakers scramble to deal with government shutdown)
Hyundai spokesman Brian Sir said the company was "confident that our Fluidic Sculpture design language will continue to be highly successful". Fluidic Sculpture is a phrase Hyundai often uses to describe its style.
Expectations are already running high that the overhaul of an ageing lineup will help Hyundai raise prices and bounce back from what is likely to be its first earnings decline since 2008 this year. Hyundai shares are trading near a record high after jumping 18 percent over the past three months, outperforming Toyota Motor, Volkswagen AG and General Motors.
The automaker is expected to report on Thursday its July-September quarter net profit was little changed from a year earlier at 2.19 trillion won ($2.06 billion), according to Thomson Reuters I/B/E/S. For 2013, profit is seen falling 3 percent, but it is likely to rebound to 9 percent next year, according to Thomson Reuters StarMine data.
The Sonata's last big makeover came in 2009, just as the global financial crisis was crushing auto sales. Hyundai opted for a bold style change, which helped it win a record share of the U.S. market and escape the industry-wide downturn largely unscathed.
(Read more: Brand rankings show promise, challenge for automakers)
But the bold look didn't go down well in Korea, its second-biggest market, where conservative styling is preferred. Now Hyundai is facing an uphill battle at home against foreign rivals, which have benefited from free trade deals and taken 12 percent of the market so far this year, up from just 2 percent a decade ago.
It could soon face the opposite problem.
"The new more conservative design might work in Korea, but it could well fall short of the company's expectations in the U.S. and China, those two key markets for volume," said an industry insider who has seen the new Sonata.
"Hyundai people think it its refined, but the car lost the identity it gained with the current model," the source told Reuters.