Okay, so here's the deal. You can substitute whatever you want from the AO recommended curriculum. Yep. It is your homeschool, and you can do whatever you think is right for your family. True. You can do what you like.
Before you do, though, please be aware that when that wonderful group of ladies who is the Advisory added a book to a year's work, they considered the character of the people, the subject matter, the era, the quality of writing, the page count and the reading level as well as whether it was a good fun book. They looked at them all in tandem.
You can combine your kids in one year if you want. You can take books out of one year and read them to three of your children of different ages. You can substitute a book because you don't like the author or because the subject matter doesn't suit your faith or your moral beliefs or because you don't like white pages with black print. You can substitute the D'Aulaire's Buffalo Bill that is hard to get for this one that you have already on Kindle. You can do whatever you think is right. When you do, though, consider what you're actually doing. What will your alteration do to page count and reading improvement? Why was this book there in the first place? Why do you want to change it? Does your book have all the same benefits that the original had?
You can also take a book out of one year because it's too difficult and put it in a later year. You can substitute The Pilgrim's Progress in the original language with one written in modern English. You can chose to read Little Pilgrim's Progress or Dangerous Journey instead. You can do this. When you do, though, remember that in Charlotte Mason's schools children of six and seven could understand The Pilgrim's Progress in the original, and ask yourself whether you really believe that the children today are less able than those of a hundred years ago.
Form IA (7 to 9) hears and tells chapter by chapter The Pilgrim's Progress and the children's narrations are delightful. No beautiful thought or bold figure escapes them.On that note, each AO year has its difficult books. Parables of Nature, The Pilgrim's Progress, Unknown to History: Captivity of Mary of Scotland, Kidnapped, A Book of Golden Deeds, Oliver Twist, Kim. You can take them all out if you wish. You can move them to a later year. If you remove Parables, though, don't expect your child to be able to understand Mary of Scotland. If you remove Golden Deeds, don't expect Kim to be easy. Books like these build muscle. Each one stretches your reading level enhancing your ability to read the one that comes after. (People tell me Mortimer Adler's How to Read a Book talks about this, but I wouldn't know. That's an AO7 book and we're just finishing up AO5.) When you remove one book, think about the implications.
Charlotte Mason A Philosophy of Education p180
Personally, I substitute lots of books because I am in Australia, and the majority of AO users are in America. I want Jemimah to know about James Cook and Hume and Hovell and bandicoots and kangaroos and platypuses and brolgas and quolls and cassowarys and quokkas and galahs. I want her to to understand the mythology of Ned Kelly. I want her to know and love Norah and Judy and Dot and Blinky Bill and Wombat and Mouse and Tabby. I substitute, but when I do, I take into account the number of pages, the readability, the reading level, the subject matter, the era and the story. I look for the types of characters that CM would have chosen in her schools. Sometimes in Australia that's hard - we don't have as many Godly role models as Americans appear to have. We do have some, though. Mostly the substitutes I find are pretty good.
If you want to change a book in your homeschool then that is okay. Don't make the decision lightly, though. Don't change something that ain't broke. Sometimes a listed book might be difficult to source. Sometimes you might not find it in your local library or in your favourite homeschool catalogue. Sometimes you might even need to part with hard earned cash. If you chose to substitute then ask yourself why. Ask yourself what the implications of your decision might be. Ask yourself if the book you've chosen is as good as the original...or even better. Ask yourself if the substitution is worth it.
If the answer is Yes, then go ahead and substitute with impunity. I do. If the answer is No, then personally I would think again.
PS Have I told you recently how much I appreciate the ladies of the AO Advisory? They're my heroes.