Personally, I don't see what would need to be changed in Magician's Nephew.

Well, I'm sure they will tone down some of the more intense scenes. Even though there really aren't any real intense scenes, they'll tone down scenes anyway. And THEN they'll add stuff that's much more intense, or scary, or violent than what was in the book, as Hollywood usually does.
I can see them trying to add action scenes. I can see them having the White Witch already getting creatures on her side. Maybe while on their way to the apple tree, they'll be attacked?
This is such a odd and yet original story IMO, that I can really see them trying to add stuff to, well, make the story more conventional to what people are use to seeing in a family movie. <_<
I really can't think of what would be changed. I do know that a lot of talking scenes will be cut out and scenes like the talking animals trying to plant Uncle Andrew in the ground will probably be cut. From what I've seen, movie makers don't seem to go for those memorable scenes that don't take the story further fast enough.
My guess is, that Jadis' world will actually fall apart and die when they leave, giving the audience a sense of dread now that they have this witch they can't take back to where she came from. Plus, it would make for a cool action scene. Seeing the apocalyptic end of the (or a) world is popular now.
Wow, look at all that I wrote. It's late here and I'm rambling on. Good night.

- Twinimage

A few thoughts that I had: Have everything take place in one day. From the moment Digory climbs over Polly's wall to when he feeds his mother the apple. Expand the London chase scene so you don't just come in at the end. Maybe build up some more tension around Uncle Andrew when the kids first see him. I believe that bit happens when it's raining, build upon that and have a big thunderstorm going on outside. And when Uncle Andrew stands up and turns around big flashes of lightning and thunder rumbling.
The scene between Digory and Uncle Andrew after Polly disappears will probably be cut down quite a bit.
I don't think Jadis already having creatures is a good idea. She was the only thing left alive on her world, so she couldn't have brought any with her. And it would appear that most of the creatures came from the north beyond Ettinsmoor and didn't actually inhabit Narnia until LWW. There weren't any of hers in MN and she goes north after the bit outside the garden with Digory. LotGK was also from the north, perhaps Jadis goes up there and corrupts a colony or something?
The other bit I can see being cut is the scene where the animals try to plant Uncle Andrew. Except something has to be done with him while Digory and Polly are off with Fledge. Hmmmmmmm. Have to think about that one.
One bit I hope they'll add though. After Digory's mother is well, have a shot of the apple tree, very small with blossoms blooming. Then transition to where the tree is bigger, with its leaves changing colors. A storm rages, the tree comes crashing down. Transition to Digory, now Professor Kirke, walking past the Spare Room. He pauses. He opens the door and looks inside. He sees the wardrobe with a dust cover on it. Cut to Lucy pulling off the cover. Fade out.

daughter of the King, I LOVE that!
I hadn't thought about the transition/link, but of course it's needed.

Daughter of the King, I think you misunderstood, I meant that Jadis will convince Narnian creatures or put some of them under a spell in Narnia to join her side. That way there's actual danger, aside from Jadis herself during their adventure in Narnia. You have to consider, the story is kind of short. They meet Uncle Andrew, go to Jadis' world, ring a bell and soon after leave with the Witch. After a brief scuffle in England, they end up in Narnia. After they arrive, Aslan sends them to the apple tree, then there's a happy ending.
The length of the story in the book doesn't feel short because of all the descriptions and narrative Lewis uses. The scenes will most likely move pretty fast, compared to reading the book, with all it's narrative. So, I can see them trying to extend scenes, and of course add scenes. I have a feeling that the first King and Queen of Narnia will probably have a quest of their own to do, so they're just not standing around. I can also see Uncle Andrew becoming more of a threat/danger while in Narnia. Maybe Jadis have some use for him and he'll assist her in whatever she's doing. Again, I think Jadis will take control of Narnia, or part of it, quicker than in the Narnia timeline. It all comes back to action and moving the story forward at a even tempo. Unless Hollywood changes in the coming years, they will still probably want to make it an action blockbuster, not an adventure film. Which is sad, cause MN will be a great film if they stick close to the book. We'll have wait to see how VDT turns out, then we'll know better on whether they are still stuck in their "we need to add big battles!" mind set.
What will be cool though, is the time period, it'll be WAY before the Pevensies, a period of time that I don't think is often shown in films. That should be cool.
I'm hoping that there's some character development for Digory and Polly. I wouldn't mind them showing how Digory's life is. They'll have to get around all that narration anyway, so there's some good screen time filler if done right.

- Twinimage

I really don't know, but I think they would expand the Battle of Charn and show it in it's full gory (PG-standard of course).

If they did make Uncle Andrew more of a threat, that would be silly. Uncle Andrew, despite his folly, was never a bad guy to begin with. Just extremely delusional and self-centered.

The best expansion areas they have are 1) Digory/Mother storyline (think backflashes), 2 Charn's history, and 3) Jadis in our world.

Besides, I think if a filmmaker paces the scenes well and doesn't either rush through them or linger too long in them, there will be little need for expansion. There are such things as good short movies .

I think i disagree that Uncle Andrew is not that bad of a guy. To send a child into the unknown and then manipulate to force a second child into the unknown, a child he is partly responsible for, that is pretty bad.

Oh, okay, I get you now twinimage.
Uncle Andrew is bad. He's just not as bad as Jadis. So when the children first see him he needs to be really creepy and awful and horrible enough for Digory to want to "punch his head." But when the children meet Jadis, Uncle Andrew seems tame in comparison. It's all about perspective.

I really don't know, but I think they would expand the Battle of Charn and show it in it's full gory (PG-standard of course).

If memory serves, something like this was discussed on the old forum. I would prefer a flashback sequence similar to the battle in the beginning of LOTR: The Fellowship of the Ring.

Aside from that, there is one thing that I can think of that they may change that I would go along with: It is when Digory, Polly, and Fledge make their way to the garden where Digory is to pick the apple for Aslan. The only dangerous moment that occurs before reaching their destination is their sensing of Jadis lurking nearby just before they go to sleep in a valley for the night.

The filmmakers will probably want to expand on this or add a bit more during this
journey. Just speculating.

Even the purest of purists know that certain changes need to happen to convert a book to film.

I do not accept this as an axiom. Adaptation necessitates creative thinking, which may, in turn, require 'changes' to translate the written word to light and sound. By no means is it, nor should it ever be, the default approach to assume 'changes' are inevitable.

I never said it should be default; I merely pointed out that it is impossible to film a book without some sort of change, no matter how insignificant that change is. It still happens.

PELS, I love MN. It's my favorite of the seven, but that won't necessarily stop someone in Hollywood from butchering it.

Anyways, if it's ever made, I think the battle of Charn will be shown and probably expanded.

ETA: I realized that I sound quite contradictory about adaptation, but I assure you I'm not What I mean is that change shouldn't be required, but I realize that it is. IMHO of course. Hopefully that makes sense.