Any popular movement that is big enough, and is willing to flex it's power against the government will ultimately win. The only question is how big it is. A government can't stand the pressure of a popular movement that is willing to exert it's power. The only defense they have, is to cave to the demands of the people.
I would think that they would make a difference, but who is to say it will be a positive one, or a negative one. I guess we will see the outcome in the future and see how the protestors make a difference. Either way there will be a reaction by their government.
I do not believe that any pro-democracy protests will make a difference in the short term in Hong Kong. The Chinese authorities will do as they please and will not be seen to bow to any pressure. It is possible that sustained and significant protests may filter their way into the thinking of China's political leaders in the decades to come, but there will be no immediate compromise by the Chinese government.
I have my doubts that the protesters in China will make a difference but I applaud them for their effort and I hope that their ability to fight forward can show others around the world that it is okay to stand up for your rights. I think it is good to see such a large demonstration. Hopefully it will change things.