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Magic The Gathering underwent this, quite subtly for the most part until the major redesign. Compare cartoonish, coloured pencil works of the first few sets with the detailed paintings of modern cards and you will see that the overall quality has improved dramatically. This is mainly thanks to detailed art style guides of each of the new sets created.
Dungeons & Dragons artwork has changed quite a lot since it was first released in 1974. The art has gotten steadily more Dungeon Punk, but this has let up somewhat in 4th Edition. A lot of fans of earlier editions do not like artwork from newer editions of D&D, and some fans of newer editions of D&D do not like the older artwork.
The artwork for Warhammer 40,000 has changed almost as much as Dungeons and Dragons over the years, starting off with goofy, oversized scifi drawings not out of place in the first edition of Shadowrun and hewing steadily more and more toward Frazetta-style apocalyptic science fantasy. Most of the combatants received major redesigns along the way as well.
BattleTech's early artwork was almost entirely simple, unshaded black-and-white, and prone to very strange/impossible Humongous Mecha anatomy. Some designs were licensed from Japanese manga/anime studios, which often looked wildly out-of-place compared the Real RobotWalking Tank designs that the game became notable for. Later editions feature shaded artwork and significantly improved anatomy, along with a more consistent art style leaning even more towards the Walking Tank philosophy (the licensed Japanese designs were redesigned or dropped due to a messy lawsuit). Art showing civilian equipment has evolved to catch up with Technology Marches On; early artwork shows enormous computers and chunky equipment (this was back in the The Eighties, mind you), while newer work shows touch-screens and other modern innovations.