This is how Stitch costumes have changed over the years.
2002-present: Since Stitch's debut in the Disney parks, his mouth is slightly closed with his upper teeth shown. It is still used in the Disney parks in Paris, and Hong Kong, Tokyo DisneySea's Fantasmic! and currently used in Mickey's Storybook Express in Shanghai Disneyland.
2013-present: Stitch's mask gets a slight modification with his mouth wide-open a bit to reveal his lower teeth. It debuted in the US parks in 2013 and Tokyo in 2015 as Stitch's double for shows and parades, but in mid-2016, it made its official debut in the resort.
Disney On Ice, 2003-2015: This is an alternate lighter Stitch costume used in Disney on Ice's Passport to Adventure. His ears are slightly bigger than his head, and looks less furry. Also, the performer bobs his head to move the mouth.