Petit has changed art styles (praise the Hayd that Petit didnt keep the Adventure art style), but never within the modern era, which is the only canon era. It's nice for the games to be "unified" under one art style, but when Petit 5 happens, should the style change?
Desert Wolf went through a similar situation. Though, the original Desert Wolf graphic was at least decent to look at. In fact, DWID referenced it in the DW4 demo he sent me. Speaking of that, the demo has a new sprite for James, so DW4 is likely to have a new artstyle, that's cool.
Whenever Tigzon is redesigned, he sometimes gets a new art style, but often, its in the style of the original Tigzon sprite, which is pretty cool.
Skyward's changed artstyles, but the old Skyward 4 demo has the Skyward Powers look, which is a good look. GOOMY, WHERE'S OUR SKYWARD 4. jk no rush..
Jingo has a lot of sprites, but most of them are in a similar style. But it seems the style will change for future games, with the sprite by DWID and the black one.