Were they already planning a Superman II with Zod while filming Superman and this was foreshadowing, or was that just a scene in the first movie that was a convenient jumping off point for the later plotted sequel? On the one hand, that was before the age of assumed sequels, but on the other, the scene was pretty jarringly superfluous to the first movie to just be there.
from the wiki page for Superman II:
In 1977, it was decided to film both Superman (1978) and its sequel simultaneously, with principal photography beginning in March 1977 and ending in October 1978. Tensions arose between Richard Donner and the producers in which a decision was made to stop filming the sequel, of which 75 percent had already been completed, and finish the first film. Following the release of Superman in December 1978, Donner was controversially fired as director, and was replaced by Richard Lester. Several members of the cast and crew declined to return in the wake of Donner’s firing. In order to be officially credited as the director, Lester re-shot most of the film with a new alternate opening and ending for which principal photography began in September 1979 and ended in March 1980.
Both films were filmed simultaneously. The sequel was planned a year before the first film was released.
As a side note, the story goes that Superman zooming around the world to change history was planned as the showstopper ending for the sequel after a can-you-top-this first movie, but they eventually did a rewrite upon concluding that, hey, if the first movie isn’t a big enough hit with the most spectacular ending we can think up, the interest won’t be there for the sequel.
In another side note, the producers had gained a measure of infamy five years earlier when they decided to make two movies out of The Three Musketeers, while paying the cast for only one movie. According to IMDB…
“As a result of producers Alexander, Ilya, and Michael Salkind splitting this movie into two parts, the Screen Actors’ Guild contracts now often feature what is called a “Salkind Clause”, which requires producers to state upfront how many movies are being shot, and that the actors and actresses involved must be paid for each. The latter clause applies even, or even especially, when producers make that decision during or after production.”
What I heard was that the two films were planned from the beginning. The original plan was that at the end of the first film, Luthor’s missiles would detonate in space, breaking the prison, and freeing Zod & co. This would be a cliffhanger leading into the second movie.
If you’ve seen Superman I and II a bunch of times and are interested, seek out and watch Superman II: The Richard Donner Cut, which was released in 2006. (Looks like it’s not currently included streaming anywhere but can be rented on Amazon Prime).
Seeing the differences respliced in and some of the silliness from 2 cut is pretty interesting.
Or I’m sure you could, in theory, just poke around YouTube for people who have called out and discussed the differences. But I’m SURE none of those have any copyright violations 
I’m actually currently rewatching 1-4, which is why the question occurred to me.
I remembered the quality falling of a cliff after the second movie, but actually it fell off after the first.
Superman III is notable for Robert Vaughn’s immortal line: “I ask you to kill Superman, and you’re telling me you couldn’t even do that one, simple thing.”
There is no Superman IV. That’s crazy talk.
This. Donner scrapped Lester’s opening scene with the atomic bomb in the Eiffel Tower freeing the Kryptonians from the Phantom Zone and put back in the scene of Luthor’s missiles freeing them from their prison. Also, and I won’t spoil it, but Lois deduces that Superman is Clark Kent instead of discovering it by accident.
What gets me is that what triggers the Phantom Zone being opened is a nuclear explosion In Space!!!. Where in space? In Space!!!. Because Space!!! is all one place.
Even as a kid, I thought that was stupid.
Jor-el: By the time you see this I will have been dead for thousands of years.
Lex Luthor: Krypton blew up on May 23 1948.
But, on the other hand, people flying and shooting heat beams from their eyes made perfect sense.
Well, yes. The authors of course know that it’s impossible for a man to fly around and shoot lasers out of his eyes. But they’re engaged in the exercise of asking, what if the world were like our own, except for the existence of people who can do things like that?. But “Space” being all one place is not a part of that fictional world they’re creating, and it’s not a part of our world, and it doesn’t explore any interesting questions, just get in the way of those questions, and it’s probably just that way because they were blithering idiots who don’t know anything about how our world (or even Superman’s world) works.
Yeah, but that’s a problem shared with most audio-visual SF/Fantasy–the aliens are either from a million miles away or a million galaxies away.
I’m still wondering why some random nuclear bomb is strong enough to break the prison, but the nuclear processes going on in stars are easily handled. 
Jor-el: By the time you see this I will have been dead for thousands of years.
Lex Luthor: Krypton blew up on May 23 1948.
Was that the date Krypton blew up, or was it the date that Superbaby and some glowing green rocks arrived on Earth?
I grew up reading DC comics in the 60s. Having a random nuclear explosion release people in the Phantom Zone is pretty typical of the wacko stuff that went on in those comic books at the time, so that part of the movie didn’t bother me.
However, the comics were usually consistent about what Superman could and could not do, so it infuriated me that the filmmakers gave Superman the ability to shoot beams from his index finger.
In the 50’s Superman could shoot rainbows and miniature Supermen from his hands.
