When the movie Abyss came out, some people told me that it would be just the same if they had put a Russian submarine in the abyss rather than the aliens they did. I could quibble about plot details, but fundamentally the whole thing would have been changed had it been other humans and not aliens. Not only the mood but the theme.
It doesn't have to change the theme. Edgar Rice Burroughs had only one plot, and it was not just that the hero got precipitated into foreign parts where he had to fight. He also met a heroine. Owing to cultural clashes, misunderstandings, and dramatic interventions, he and she did not get together until the end of the book. When he dropped the heroine, the story did not work, partly because it didn't have form. But when he tried to have the hero chase after the heroine whom he had already won, and married, and perhaps had children with, it did not work, either. The form of the story was the same, but the mood of rescuing your wife needs to be different from the mood of rescuing a damsel in distress who may or may not be in love with you, and may or may not marry you. . . .
It's amazing what you can do with theme without altering the plot much, if at all. In Romeus and Juliet, the moral is laid out for us in advance:
And to this end, good Reader, is this tragical matter written, to describe unto thee a couple of unfortunate lovers, thralling themselves to unhonest desire; neglecting the authority and advice of parents and friends; conferring their principal counsels with drunken gossips and superstitious friars (the naturally fit instruments of unchastity); attempting all adventures of peril for th' attaining of their wished lust; using auricular confession the key of whoredom and treason, for furtherance of their purpose; abusing the honourable name of lawful marriage to cloak the shame of stolen contracts; finally by all means of unhonest life hasting to most unhappy death. This precedent, good Reader, shall be to thee, as the slaves of Lacedemon, oppressed with excess of drink, deformed and altered from likeness of men both in mind and use of body, were to the free-born children, so shewed to them by their parents, to th' intent to raise in them an hateful loathing of so filthy beastliness.
Which you may not quite recollect as the moral of Romeo and Juliet.
I've got a story that decided it doesn't have enough emphasis on the heroine's internal conflict. Not that she didn't have that before. But it needs sprucing up, and many moments to be made more dramatic and terrible --which is going to change the whole mood of the piece.