I have the same setup and when I am in windows, it offers the free upgrade from windows 7 to 10, but when I try to download the upgrade it never actually does.
I did download an iso image of win 10 on my own but I have no idea how to install it using bootcamp like I did with windows 7.
You might want to try to download the iso of windows 10 before the free upgrade deadline and then try to figure out how to get it working on the mac pro.