Sometimes it is simply, he’s the boss and there’s the door. Your choice.
I’ve learned this a long time ago: You work for a company. The company owns whatever you’ve done. The company decides the fate of this work.
While it’s true, you’re part of the company, it really depends on the boss and what type of person he is in terms of any successful negotiation with him.
This is something that you can get a good sense just by talking to him a few times. Is this a reasonable person? Will this go well?
But even before asking such questions, you should try to be as objective as possible and make sure you understand the new direction and its impact. Perhaps your emotions got the best of you and the new direction is actually a good one? Sometimes our egos get the best of us.
If in fact the new direction is a bad one, now its time to decide if you want a discussion with the boss. In so doing, I would try to be as tactful as possible. Anything that starts with “You’re wrong…” or “This is stupid…” won’t fare well. Usually its about tact and charm and not everyone have them.
I would first try to make a compromise if I go on this path. This would be a win-win for both parties. Everyone gets their idea injected and everyone is happy.
However, if you feel so strongly that its your idea or nothing then that is also a path you can take and it may work depending on the type of boss.
In the end, you will find its less about the actual idea and more about how you navigate the politics and deal with others.