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 those of Season 4. A notable example occurred in the Season 4 premiere, in which 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.
This gets lampshaded hard in the Mirror Universe two-parter "In a Mirror, Darkly", when the Terran Empire version of the NX-01 crew discover the Constitution-class U.S.S. Defiant from the TOS episode "The Tholian Web". Cue Mirror!Archer and company showing their amazement at the "advanced" technology.
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.
Speaking of Doctor Who, the graphics used for things like the Time Vortex, death rays (from future weaponry, Daleks, or Cybermen) and spaceships have evolved over the years. Dalek death rays nowadays show a CGI three dimensional X-ray of the victim's skeleton. During the 1960's and 1970's the whole screen would go negative and in the early-mid '80's the Daleks made only their victims turn negative when they shot them. "Remembrance of the Daleks" (1988) showed for the first time a comparatively crude CGI-free version of the skeleton effect, but they could only afford to show it once. The same serial was also the first time Daleks hovered up stairs. "The Ark in Space" (1975) used a model for exterior of the titular ark, and the DVD (released in 2002) has an option to replace these scenes with a CGI version.
Recorded 911 calls are presented on World's Dumbest... with animation. Early seasons show rough and unpolished MS Paint art with simple animation, while recent seasons have lavish and almost cartoon-quality Flash animation.
Both Kamen Rider and Super Sentai have gone through a significant Art Evolution. Since both of these series are around for a very long time, suit designs, props and special effects have improved considerably. Just compare the earlier Transformation Sequences in Super Sentai to the later ones in this video.
This trope is also seen with the character of Apollo Geist. When he first appeared in Kamen Rider X, he wore a simple suit◊, without much detailing. When the character returned in anniversary series Kamen Rider Decade, his suit◊ was more detailed, sleeker and more modern looking compared to his first appearance.