If I erase my hard drive on Yosemite, will it stay on Yosemite or convert back into the factory OS (Snow Leopard)?

I am trying to resell my 13" Macbook Pro of 6 years. I would like to erase all of the data on it currently. I have discovered how to do this by going into Recovery mood and then erasing the partition. However, when I bought my computer, it was running on Snow Leopard and I have since upgraded to Yosemite. How can I erase all of my data and make sure that Yosemite stays when I sell it and it doesn't retreat back to the factory OS? Thanks in advance!