Changing MSc supervisor late in the game
Switching now seems like a poor choice. You can finish in a bit, you say. And then you can change areas. Your old professor is happy with you. You have a success under your belt. The world is open to you. 

Anyone can change at any time. There is no real reason to waste your past work. 

You can then take up, somehow, in the new area, perhaps with the other professor. Or, maybe even apply for a doctoral program that will support you in the new area. 

Being in an area with only a few people is fairly common in some fields (mathematics, say). It is a tradeoff. There is less competition, for example, though you have to look harder for collaborators. 

Don't think that finishing in the current track will lock you in to that forever. You have valuable experience that you can exploit in a lot of ways. 

And even if you do switch, abandoning old work, don't make a habit of it. Throughout your career other opportunities will sometimes seem pretty bright. If you chase every one you won't ever accomplish much. Finish what you start unless the situation is so bad as to have negative impacts on your life. That doesn't seem to be what you say here.