transfer unlimited data plan

I have an ipad 3 with an unlimited data plan that I got when i bought my first ipad. unfortunately i shattered the screen and the ipad still works but not very well. I also have a functioning ipad 2 with no plan. Since i plan on buying the bigger ipad expected out this fall, i dont want to spend the money to repair the ipad 3 but would like to transfer my plan back to my ipad 2.  how can i do this?