Was this really the original American/English-language opening/intro to Sailor Moon? If so, why was it so heavily changed?
I'll preface this answer by admitting that I couldn't find any interviews in which DiC (the company responsible for the original English-language dub) explained why they changed the opening so much. I've nonetheless tried to provide the best explanation possible for why it was done.


Was this really the original American/English-language opening/intro to Sailor Moon?

Sure was, and for those who grew up with it, it's arguably just as iconic as (say) the original English-language Pokémon intro.

Why did they so heavily change the already perfect intro sequence and opening theme like that?

Because that was the done thing. Back in the 90s (and in fact, up until quite recently), it was common practice to &quot;Westernise&quot; imported shows as much as possible, to make them more familiar and relatable to Western audiences. This included replacing the Japanese intros and outros with brand new ones - in English, of course.
According to Wikipedia, the Sailor Moon intro was actually unusual for the time in that it kept the melody of the Japanese intro and just changed the lyrics. Most dubbed anime of the time just threw the intros out entirely and made new ones from scratch (see the Pokémon example above, and also Sonic X, Yu-Gi-Oh, One Piece... basically anything 4Kids got their hands on).

Wasn't the point of importing this very Japanese show to the West to keep the look, sound and atmosphere, rather than changing it all around?

Nope. The point of importing it was that it was insanely popular and it was going to make a lot of money. As noted above, keeping the Japanese atmosphere was not only not the point, it was the complete opposite of what actually happened, not just to Sailor Moon but to pretty much every anime that made its way to the US until about the late 2000s.

I'm not sure, but I believe that they didn't touch the show itself.

I'm afraid they did. Changes included:

Giving everyone American names instead of Japanese ones (Usagi Tsukino -&gt; Serena, Ami Mizuno -&gt; Amy Anderson, Mamoru -&gt; Darien, etc.)
Adding &quot;Sailor Says&quot; segments to the end of every episode in which Serena gives the viewer life advice (example)
The removal of any hints of nudity or violence against children (and yes, probably the pantyshots)
Making Uranus and Neptune cousins and removing any romantic subtext between them
Making Zoicite a woman so their love for Kunzite/Malachite wouldn't be gay

And probably a whole load of others. Mind you, Sailor Moon got off pretty lightly compared to Kyoryu Sentai Zyuranger (from which the fight scenes in the original Power Rangers were taken; they threw literally everything else out and filmed their own plotlines with Western actors), and the three different anime that were torn apart, mashed together, and re-dubbed to create Robotech.

[Why would they] introduce characters from far into the series right away?

I can't answer that, but spoiler openings are ubiquitous in anime. Even the original Japanese intro(s) contained one or two spoilers.