The remake of Battlestar Galactica received a number of visual upgrades throughout its run. The easiest way to tell is by comparing the Cylon Centurions from Season 1 with the Centurions of season 4. A notable example occurred in the season 4 premiere, when a certain epic battle looked notably more epic than it might have in seasons past.
Star Trek: Enterprise, being a prequel series, ran into the problem that it was being produced over 30 years after Star Trek: The Original Series, but was supposed to be chronologically set 100 years before it. The set design and costume departments have a lot to say about their struggles to make it look like a logical evolutionary step between a modern-day NASA space shuttle and Kirk's Enterprise from the Original Series...even though 1960s designs look very backward now.
In an unusual example, the sitcom Reba. Earlier episodes used standard establishing video shots of the characters' house. Around Season 2 these were replaced with warm-filtered, soft-focus zoom on stills of various set details.
The three CGI-animated "hosts" of the German children's Edutainment ShowWas ist was TV went through a slight redesign between the show's two seasons. The original designs◊ (note that Quentin the period is seriously Off Model here) may have looked simplistic, but demonstrating that this trope doesn't always yield attractive results, the redesigns have a way more uncanny feel to them, especially since these new models have their lips stuck like that.
Disney Channel sitcoms used to be shown in full-screen and were shot on tape, but soon switched to widescreen and became shot on film (though it's still video technology).
The cartoon version of Lizzie McGuire at first looked crudely drawn, but the animation soon improved.
Recorded 911 calls are presented on World's Dumbest with animation. Early seasons have the MS Paint art rough and unpolished with simple animation, while recent seasons have lavish and almost cartoon-quality Flash animation.