Mysteries are incredibly flexible. They can bend and twist to fit any circumstances or situation. If you are writing an interactive mystery event, you just have to know what you have to work with in the beginning to save yourself the work of adjusting elements later on. If you are writing a book, you have much more freedom – but you still need to know where you are going to end up. One of the great joys of being a writer is being able to lay the infrastructure so that it supports all the twists and turns you are going to pull your reader through on their way to solving the crime.
This does not mean you are married to a rigid outline or structure. Yes, you do have to make one, but if along the way a better idea pops up, follow it as far as you reasonable can. If this bright new idea is asking you to rearrange just about everything – it’s probably a different book. If you can easily tweak existing ideas to support the new twist – by all means follow it if it improves the story.