Yes, people's minds can be changed. However, most likely, the mind's that will be open to change are typically thos within the "fat middle". These are the individuals who are honestly either not particular to either party OR are just relatively light supporters or one or the other. These people tend NOT to be individuals heavily engaged in politics or the type of join a political forum. These are your average individuals whoh at best pay attention to politics a bit during the election season and at worst will possibly talk about politics amongst some friends but even that is generally at a very base or individualized level.
Politically minded people tend to be firmly rooted in their ideas and ideology, shifting a bit here or there but generally knowing full well where they stand, how they feel, and what they think. They make up the minority. For many people, Politics is not as in depth as many of us here view it. I think those are the type of people whose minds could still be changed. Those individuals are just barely beginning to likely really pay attention to the election stuff, if at all, and the full cycle hasn't really begun yet.
I don't think you're going to find a ton of people on a forum like this who HONESTLY could say they don't know who they wll vote for. Most likely, in a place like this, what you have is someone saying "they don't know" but in reality they have a good guess of who it is that they're LIKELY to vote for and whether there's an honest and legitimate shot to go the other way. I think the people whose minds may be swayed one way or another are those who will only really make their voices heard much in the political realm on electoin day.