In my opinion, since you advisor is meant to be a guide for your work, you should be able to choose one, at any time, based on how your work needs to be guided. I am interested in whether you chose this person or if he was assigned to you. Regardless, though, I think you should perhaps talk to the dean and see what he/she says. I would ask if other ma students in the past have changed advisors, especially this late into the process.
Another thing I might say, though, is that it might be helpful to take a step back and look at why your advisor is suggesting such fundamental changes. Are these changes based on any bias on his part? I think it might behoove you to take another look at what changes he suggests and why he suggests them, even though I am sure you have already done so. Maybe take another look. This sounds like a tricky situation all around, though. Good luck.