If you filed a complaint, and then were threatened with a wage reduction and demotion as punishment for doing that, and you refuse this and voluntarily quit the job, your chances are fairly good that you would be approved for unemployment insurance. Because this is a pretty good job related reason to quit, just on the surface of the thing. It would depend somewhat on how severe the pay cut, how much lower in hierarchy the new job offered.
If they FIRE you for refusing the new job pay and description, you've a much better chance of being approved for unemployment. If they say, "You're either going to take this new job, or else you're fired." jump on it.
Several things will come into play. The reason you filed the complaint. Whether there was a stream of discipline against you on going for any reason. Whether on-going disciplinary issues were related to the complaint you filed (for example, if you were being written up for tardiness and you filed a complaint that your supervisor was picking on you unfairly by always writing you up for tardiness)
So we really can't say much until we have a bit more information.