Having recently had access to a copy of the 1971 original The Carpet People, I can answer that the changes are pretty extensive. There are lots of little changes too numerous to mention, so I'll stick to the major differences. (Since most folks will be familiar with the 1992 version, I’ll concentrate on how the original differs from it, rather than trying to summarise both versions.)
Most importantly, the original is not a comedy - it's a straight fantasy. There are almost no jokes, and it feels like an old-school adventure in the Tolkien mould. This affects everything from the dialogue, which doesn't sound much like later Pratchett’s plain speaking, to the plot, which while broadly the same lacks subtlety and relies on convenience rather than the cunning of the characters. There are no asides where characters question the status quo, they don't engage in witty banter, and we rarely know why things are happening or why characters make the choices they do. Some of that isn't too unusual for middle grade fiction, though, and it's important to note that even the original was written with a younger audience in mind.
Significant plot differences include:
- The wights are mystics, and very similar to elves, described as the "original" people of the Carpet. Pismire even refers to the fact that the days of them visiting settlements of other Carpet People are long over, and that the wights might soon "leave" the Carpet, though this is not followed up. They also don't have the memories of the future possessed by the wights of the '92 version, just mysterious knowledge. The Thunorgs are not oddities born randomly amongst the wights, but instead a separate people who split from the wights in the early days of the Carpet; the wights are said to have tried to understand and master the Carpet through knowledge, while the Thunorgs - who come across a bit more like ents - became one with it, and have mystical power to control and commune with it.
- As a result of the above differences, the meeting with the wights in Chapter IV is very different. It resembles the meeting of the Fellowship with Galadriel in that each of the seven who attend the banquet are given a gift of their heart's desire, divined by Noral's daughter Shenda through a ritual dance.
- Bane's true identity as an exiled General of the Dumii is revealed during Chapter IV, though Snibril still doesn't get the full story until he meets up with the fifteenth legion in Chapter XVII. Bane is also revealed to be in love with Noral’s daughter Shenda, and marries her at the end, after spending almost the entire final battle unconscious thanks to a poisoned moul arrow. His backstory is also a little different, and given more detail: he was still exiled after killing a would-be assassin, but in the original it's because he pursued the assassin onto the roof of a temple and broke a taboo against killing on holy ground. The council debates his fate and eventually decides instead of the usual lifelong exile, he will be exiled for seven years and stripped of his title; he doesn't contest the ruling and leaves, even though he was very popular and probably could have deposed the Emperor in a coup if he'd wanted to. The Emperor later regretted this, and sent soldiers to find Bane, but they never found him; there’s no mention of the Emperor changing the law as in ‘92.
- Glurk and the others don't meet Culain (who is written as male in the original, and not named Culaina) until Acretongue takes them to the sugar crystal in Chapter XIV. Instead of having met Culaina and being guided by her, Glurk gets lucky when he enters the High Gate Lands by waylaying a sympathetic Vortgorn, who lets him take his armour and tells him about the pomes and where to find them.
- The denizens of the High Gate Lands, the Vortgorns (also called Vortigorns in various places), play a more major part; rather than just subjugating them in order to mine copper for superior weapons, the mouls force them to fight with them, and they appear in the final battle.
- Ware has not been infiltrated by mouls in the original; they are recalling their armies because they are aware of the threat of Fray and the mouls, but many of the legions have not made it back or have already been killed by mouls marching towards Ware. (An encounter with a mostly destroyed outpost foreshadows the Thunorgs’ involvement in the finale.) The Emperor has no son, and is still alive, but we don't meet him; he is killed along with his courtiers when Fray first strikes Ware, trapped in a collapsing temple.
- Once the moul armies are outside the city walls, Bane is struck by one of their poison arrows and nearly dies, remaining in a coma until after the final battle is done. They don't meet with the moul leaders to try and negotiate; instead Snibril sneaks alone into their camp and returns with the Vortgorn leader, Stagbat, as his prisoner. Eventually Stagbat realises the mouls can't control Fray, and leads his people to rise up against them, helping to turn the tide of the battle.
- Speaking of the final battle, it's won thanks to the help of other peoples turning up to assist: the wights (led by Athan, who vanishes with his group immediately after Snibril saves them, rather than travelling with him for a time), the Vortgorns (as described above) and the Thurnorgs, who bring a horde of Carpet creatures with them and who cause the hairs of the Carpet to crush the surviving mouls who flee at the end of the battle. There's no mention in the original of Mealy and the mess hall sergeants, or the women armed by Brocando. (Indeed there are almost no women in the '71 version at all; only Glurk's wife Bertha and Noral's daughter Shenda are mentioned by name.)
The protagonists were all changed for the '92 version, too - generally to be more canny and modern in their outlook. In the original:
- Pismire is a Gandalf-like sage, described in his introduction as a "magician" rather than a shaman or "odd-job holy man". He has no backstory - he doesn't seem to be one of the Munrungs, but it's not explained where he comes from or how he came by his knowledge.
- Snibril seems to be chosen as a hero by the Carpet; he finds himself suddenly possessed of courage and physical strength in the middle of battles, and fortune shines on him by providing weapons at just the right time. This is most obvious in the fight against the snargs in Chapter II, where he single-handedly defeats Snarfgorm, the King of Snargs (changed into a generic "very big snarg" in the '92 version). But he's also a much more straightforward heroic character, with very little of the "reluctant hero" or “there must be another way” attitude of the '92 version. He doesn't question things nearly as much, nor does he make any impassioned speeches, aside from being the mediator at the peace treaty of the various peoples after the battle.
- Both Glurk and Snibril are much luckier in the original, and less witty. They succeed through being tenacious and having good fortune as much as anything else.
- Bane's backstory and fate are different, as described above, and he is also still loyal to the Dumii empire. He overall feels like Aragorn, though weirdly absent from parts of the story.
- Brocando is largely the same, but without the layer of parody of the modern royal family added in '92. He is also a little more noble in the original; for example, he stops the deftmenes from killing his cousin on his own, without needing to be convinced by the Munrungs.
- Careus is named Carus in the original, and is a General rather than a Sergeant. He makes Snibril a Captain in the Dumii army and puts him in charge of the Munrungs and deftmenes who are with him, and as the only senior military person still alive, helps Bane command Ware after the Emperor’s death.
- Owlglass is still an old friend of Pismire, but has a larger role, as the pair look after Bane when he is wounded by the moul arrow.
So overall, it's a much straighter fantasy, with fewer hints as to what things in the Carpet are from a human perspective, less introspection, way fewer jokes, and none of the older Pratchett's disdain for authority, war and tradition. It's a fun romp and a good read, but it's not a patch on the '92 version in terms of laughs, attitude or philosophy.
Last, but not least: the original has multiple illustrations by Pratchett throughout. The originals are mostly black and white line art, with a few that are fully shaded. Many of these drawings appear in an illustrated edition of the book published in around 2005, I think in colour, but that edition uses the 1992 text and so only includes the ones that are still relevant to the new version. (It’s known there are a few rare copies of the original edition in which Pratchett hand-coloured some illustrations, and a couple where he did so for all of them; those are exceedingly rare.)