You don't need me to tell you that that's a long time. And people change over time. Priorities change, life circumstances shift, and the person you were when you set out writing that book is long gone by the time you write that last sentence.
It's one of the many reasons why it's recommended that your first draft should be pulled out of you within the space of mere months: so it gives it a sense of a unified voice and tone.
But it's inevitable that sometimes you will change course. The message of your book doesn't ring true for you any more, or quite frankly you've come with an even better idea for the course of the plot than the one you've started.
So what to do? Do you stay true to your original vision, or do you chart a course into new waters?
It's a tough question, one with no definite answer or rule. One thing that we can all agree on is that your draft shouldn't be at the mercy of whatever mood you're in that day. Some of the less-polished novels out there have open seams for all to see where you can see with your own eyes where the author put down their book for a couple of days then picked up the pen again with an oh-so-subtle but distinct new voice. Moreover, in certain books you can almost see the author having an epiphany as a fantastic new idea dawns upon them as they shoehorn in a fantastic new plot twist or a brilliant explanation that explains every plot hole up until now.
But what about those genuinely good course changes? The ones that make sense and would ultimately benefit your novel? I think it depends on two factors: the base story, and what type of course-changing idea you have. There are plot-based stories and ideas that dictate the direction of your story, and then there are tone-based stories and ideas which envelope the story in a certain feel and will guide the reader towards a certain conclusion on the book's meaning, whether intended or not.
Basically, if have a plot-based story and have a plot-based course changer, you should be able to integrate that change without much trouble, provided you reverse-engineer your plot appropriately so it doesn't appear that your new twist appeared out of the blue.
For ones that are a mix of each (so plot-based stories with a tonal-change or vice-versa), you have a little more of a headache on your hands, as you might need reverse-your novel from the start. Tonal-changes mid-novel are possible but they can't appear out of nowhere. Foreshadowing is the key.
Finally, if you have a novel that is all about the atmosphere and inspiring a certain emotion in your audience, then you should very, very carefully consider the consequences of changing that mid-novel. With a course changer like that, you're basically talking about writing a new novel altogether.
And that's the thing: you shouldn't have to write a book that you don't want to write, but there is a benefit to writing a book that remains true to its vision from beginning to end. There's a purity and tightness to it: a novel sets out to do something, does it without wavering or taking detours, the end. Don't sell yourself short on just how valuable that is.