There is not a tested method to repartition your SD card without losing data. I’ve doing some research and most of the things I’ve found so far are for Linux, but I found this method for Windows OS that you might find helpful.
http://rockgadgetnews.blogspot.mx/2011/12/tutorial-how-to-partition-your-sd-card.html. Please let us know your results and if it worked for you.
I am trying to use Gparted USB and I resized the partition with it successfully !