Brand new idea suddenly changing everything?

Go with it.  It will always be better than the idea you planned.  Why?  Because the story is live, real and working.  The characters have settled in and grown.  Your story impetus is probably quite different from the way you envisaged it.

The trouble with outlines and plots is they are just ideas.  Once a story takes off it has its own momentum and you need to allow for change.  Usually it's not that radical, but sometimes you will suddenly have a brainwave which entails taking the story in a whole new direction.

This is quite scary, to be honest.  I've just had this happen - hence the blog.  I was about 15,000 words off my target, had taken the story along the lines I intended and was pushing it towards the climax.  Then it happened.  A picture jumped into my head, along with the concept of a totally different climax.

OH NO, I said.  This means I've got to change direction.  How on earth am I going to get to that instead?  I looked at it for a bit, and the concept started to grow and I knew I had to do it that way.  It was miles better than my original plan.

I wasted a couple of writing days because I needed thinking time to figure it out.  I struggled with the first couple of scenes once I went back to writing.  And then it started to flow again and I'm back into it and can see exactly how to get there.  I can pick up loose ends earlier in the book during revision.

With the last book it happened too - the identity of the murderer suddenly changed - but that was earlier and more easily incorporated.

You just never know when it's going to hit, but when it does, it's best to run with it.  If you don't, you'll find yourself writing against your instinct and that makes life very tough indeed.  Why hang onto an idea that just died?  Use the live one and your story will be the more alive.

A word of caution.  This does not mean endless replotting when you're writing Chapter One!  You'd be well into the story if one of these was going to hit you.

Trust your instinct.  Your subconscious threw this up at you.  Grab it and use it.