So you're writing along, you've got a good story line, your characters are being (mostly) cooperative ... only, you've decided that maybe the story would be better if you gave Sadie a dog instead of a cat ... or if Jack were a Jill instead ... or maybe you have a few too many characters and need to get rid of a few ... or maybe the story would sound better in first person ...
In other words ... you need to make a change or two.
This is a perfectly legal thing to do. Your book is your book, and you can do whatever you want to it. Although, I will warn you, some of your characters may resent some of your changes, especially if you decide to get rid of, say, a close friend, or a close sibling, their love interest ... or them. However, your story is yours to change, and they can't really do a thing about it.
However ... if you do decide to make a change, you'll need to go through and make sure that your story is consistent with the change. If you turned Jack into Jill, make sure you not only go through and make sure that you have changed the name every time you mentioned him, you also change all of the him's, he's and his's into her's, she's and hers's. If you change Sadie's cat into a dog, you need to make sure that you have her dog chase cats and cars, rather than sit in laps and purr. If you get rid of a character, you need to make sure that you remove all mentions of that character, and that another character takes care of everything that the character you got rid of was supposed to do. If you don't make sure that the change is made consistent, your readers WILL notice. Trust me, readers do that sort of thing.
If your change is big enough, you may want to start completely over. Get a new notebook, or a new computer document, and start over. This is probably the better option if you got rid of more than one or two character, changed the person, or made a major change to the main character, or added a new character who is needed to be there from the very beginning.
Now, if you have a character who is unhappy with the change, you'll have to tread carefully. They will probably be very resentful in your continued writing, and possibly uncooperative. Be nice to them, and see what you can do about giving them something to make up for the inconvenience you made them.
Now, that's all I have to say on that topic. You can make changes, big changes, but make sure you keep them consistent. If you have any opinions on this topic, feel free to comment, and if you have a request for me to write on, you can leave a comment with that request.
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