For its first 23 issues, it was an EC comic book, published in full color with real ads, though the magazine adopted both some five decades later. Stories in the earliest issues were often only slightly sillier takes on typical EC Comics scenarios, though the focus soon shifted to parodies of popular shows and cultural trends. Alfred E. Neuman was not yet the mascot, but made a few unnamed appearances.
The early issues (of the magazine format) were very different. The humor was "lighter and softer", the TV/movie satires were less biting, politics was rarely spoofed, and most notably, they had contributions by famous humorists of the day (Bob & Ray, Danny Kaye, Sid Caesar, Andy Griffith, Stan Freberg, Tom Lehrer, etc.). Some articles were long essays with minimal art. It wasn't until the sixties that Mad gained the format it's most known for.
Some individual artists displayed this. Early on, most of the artists tried to copy Will Elder and Harvey Kurtzman's style before setting off in their own directions.
Don Martin's early work was more Black Comedy, with little of the manic pacing and Written Sound Effects that he would later become known for. His change in style was likely due to Duck Edwing later joining him as a frequent ghost-writer.
And even that changed again in the late 1990s, as the magazine became more PG-13 with often darker humor, more profanity, Toilet Humor, etc. This change caused some of the stalwart artists like Jack Davis to leave.
Some of the first Spy vs. Spy comics, including the two oldest ones, feature the Black Spy and White Spy foiling each others' schemes without killing each other. Also, it's gone from the typical "cartoony" violence (used by original artist Antonio Prohías and his successors, Bob Clarke, George Woodbridge, and Dave Manak) to the nearly Happy Tree Friends levels of Ludicrous Gibs often employed by Peter Kuper, who took over in 1997.
Goofus and Gallant (from Highlights for Children) were originally drawn in a Brothers Grimm-esque setting with pointy elf ears. By the mid-1950s they had become human children in a contemporary backdrop, but there've still been many, many Art Shifts since.
The first three years of MacAddict were far more colorful and playful, with plenty of tongue-in-cheek humor, Running Gags and even themed comics. Over time, Cerebus Syndrome set in quite hard. The magazine got a sterile white-and-blue makeover, lost most of the cartoony edges, and was renamed Mac|Life in 2007.
Going the other way, Disney Adventures was a lot less colorful in its early years — there were fewer comics and the overall tone was more serious.
Motoring magazines are often a victim of this trope:
Auto Exchange, a free publication launched in 1997, had "THE BEST SELECTION OF USED CARS" and "SAY YOU SAW IT IN AUTO EXCHANGE - TEL: 0000 000 000" as its header in Arial/Helvetica font, and covers that looknaff by today's standards, until 1999 when it found a better direction.
Go and look at a back issue of Time from, say, the 1950s. You'll notice the overall format is pretty much the same as it is now, but the articles are much shorter - especially the arts and entertainment features - just as they would be in a newspaper (and, in fact, for the first several decades of its existence, Time billed itself not as a magazine but as a newsmagazine). It also had illustrations instead of photographs on its covers until about the 1980s.
Heat is one of the best-selling celeb/gossip/beauty/fashion magazines in Britain. However, it originally launched in the late-90s as a more serious, general entertainment-focused magazine, described at the time as a somewhat hipper alternative to the RadioTimes. Despite a positive reception, it didn't sell well, and quickly shifted to the present reality/soap-celebrity-focused, female-oriented format.
FHM was originally a quarterly magazine focusing on men's fashion and lifestyle. It later switched to a monthly release and after realizing covers with scantily clad celebrities on them sold way better the format changed to the lads' mag everyone is familiar with these days.
Brazilian magazine Mundo Estranho started out as a series of compilations of several articles from another magazine, Super Interessante, before developing its own identity.