Hey, remember pogs? How about Troll dolls? If you don’t know what I’m talking about, don’t worry. It’s only a matter of time before someone tries to bring them back. The same is true for our entertainment. Whether talking about America or Japan, the last several years have been rife with remakes, reboots, revivals, relaunches, or anything else prefixed with “re-” to let you know it’s happened before and is happening again. Some take this to mean “people are either lazy or out of ideas” or “these people just want your money by 
kicking out yet another edition of something that was/is popular.” Incidentally, the title of the final Rebuild of Evangelion movie will be Evangelion 4.0: You Will [Not] Stop Buying Rei and Asuka Merchandise. Scout’s honor!
But remakes aren’t inherently bad. I still say that Giant Robo: The Day the Earth Stood Still is the best OAV ever made, and technically that’s a reimagining of the live-action Giant Robo series (aka Johnny Sokko and His Flying Robot). So let’s take a look at a few notable anime ones; bear in mind the list is by no means comprehensive.
The most common intent behind the remake is to introduce a new generation to an older title, with the trick being to update it for the modern era without changing it so much that it loses what made the original good. Confusion tends to set in with new fans as they often assume that you need to see the original to make sense of the new one, but that is almost never ever the case. What better example is there than the first anime TV series with a continuing storyline, good ol’ Astro Boy himself? The original 1960s series was in black and white and was notoriously low budget. Pretty tough sell for modern audiences, so twenty years later in the 1980s it was remade in color. A slightly edited DVD set of this series was released in the US, and you can watch the dubbed version of this online via Netflix and such. In 2003 Astro Boy was remade once again as what has got to be one of the single most expensive TV shows ever produced, which is why it’s so sad that nobody ever fansubbed it and the US DVD release is cut to ribbons (cropped from widescreen to pan and scan, scenes removed, all music changed, you name it). Both of those remakes are highly recommended, with the 1980s 
version being most faithful to the original manga which is a lot more serious and thought-provoking than one might initially think. The new Astro Boy movie set for release in US theaters in a few months? I got no idea, man. Other Tezuka properties that have been remade in the last few years include Phoenix and Black Jack, and you needn’t worry if you’ve never seen any of the originals.
Some remakes are born out of a need to accomplish what the previous work did not do. In the case of anime, that usually means “stick more closely to the original source material” because a great many anime series are adapted from manga/novels/games that are themselves unfinished at the time of production. As a result, the anime creators have to write their own endings, with wildly varying results. I prefer to live my life pretending that Kanon simply doesn’t exist, so I’ll say that if you want great examples of this then look no further than Hellsing Ultimate and Fullmetal Alchemist: Brotherhood. If you want a not-as-great example, the recent TV series for The Guyver is more faithful to the manga which means it’s not as ludicrously violent as the original anime. For these types of remakes, seeing the original is not only unnecessary but discouraged!
There may be not much of a nostalgia crowd for older anime here in America, but in Japan there are enough of those people out there to warrant making new editions of older titles under the adage “if it ain’t broke, don’t fix it.” Since so much anime throughout the 1970s and 1980s were mecha series, a lot of the nostalgia-seeking remakes tend to involve robots. Fans today tend to look at mecha anime the same way as Westerns are looked upon by moviegoers: a dead genre whose new entries are meant primarily for older audiences. 
Luckily, I like Westerns and giant robots alike! Go Nagai and his studio Dynamic Planning seem to live and die by this crowd, giving rise to new versions of Getter Robo, Kotetsushin Jeeg, Mazinger Z, and such. One or two of these MIGHT assume you’ve seen the original show, but in all honesty the premises of cartoons back then were simple enough that you can figure everything out lickity-split.
I wonder: as the years go by, does that mean we’ll someday be seeing remakes of harem comedies from the 1990s? I’m not so sure. For that crowd it seems as though “out of sight, out of mind” is the order of the day, but in the unlikely event that the genre evaporates almost entirely, remakes of To Heart await us.
They say that for every ten anime series produced only one is successful. In light of this, you can see why studios frequently decide to remake a previously popular title. It happens so often that even if I were to just talk about the good ones, I’d probably never be able to list them all. So don’t get too mad that I didn’t mention Casshern Sins or the myriad of 90s remakes such as the mighty Tekkaman Blade. And if you’re wondering why I didn’t mention the various remakes and continuations of Fist of the North Star in recent years, it’s because those deserve their own dedicated articles!