It obviously is too late now to pick right up where they left off, with the same actors. Will Poulter is what, nine feet tall by now? But I nonetheless would support a continuation with whatever actor substitutions were necessary. Great though the familiar actors are, the STORY is the star. My only worry would be if Andrew Adamson got his hands on the next movie. Then Puddleglum would be depicted as a complete incompetent who couldn't find his nose with both hands unless his brilliant nine-year-old sister showed him how.
Let some time pass, and we'd be able to re-use at least a couple of actors. Picture "Magician's Nephew" with a twentyish Will Poulter as Frank the cabbie, and Georgie herself as his wife Helen.
I see what your saying. Yes the story is what makes us read the books and watch the films. I wouldn't want the films to go the way that Harry Potter did, and stray too far away from the original source material. Filmmakers should respect the authors works when adapting a book, but it doesn't always happen that way, no matter how hard we wish it.
The final scene between Harry and Voldemort, the battle in the Ministry of Magic in Order of the Phoenix, a lot of Goblet of Fire differed from the book. There's so much more that I can go on about.
Yes, they did made changes, but most of theses were more about cutting down content so they could tell the story in two and a half hours (or however long the movies are)and to help translate the story form book to film. But the changes in Harry Potter were small time compared to what they did in Prince Caspian and Dawn Tredder, where they changed major plot points, such as added the mist and sword or the raid on Miraz's castle, which changed the whole story.