There are a few options however the best solution seems to be using Paragon Camp Tune, which is a piece of software that will automate the entire task for you for the low price of $24.95. Alternatively you could us a Free program called Macrium reflect, which you would install on Windows to begin with, then you would want to make an backup image of your entire Windows OS to a backup. Thereafter you can create bootable media using Macrium Reflect. Then you would go ahead and remove you old instance of Window, then re configure Boot Camp for a fresh install of Windows.
Once you have re configured Boot Camp partitions you would then boot up the Macrium Rescue media you previously created and then restore your Image of your Windows OS you also previously created.
I would go with the Paragon software, as it will save you much time.
Let us know if you have any questions or if you are able to accomplish the task at hand, best of luck.