Hi,
My suggestion: clone the existing HDD to a new HDD then install new HDD on your machine before the upgrade. Doing this way you won't have to spend time to reinstall applications and all files/data. After that, you can use the old HDD as a second HDD or put it in an enclosure and use it as an external HDD.
There are many clone software around, including free software:
http://www.techrepublic.com/blog/five-apps/five-free-and-reliable-cloning-tools/
http://www.techsupportalert.com/best-free-drive-cloning-software.htm
Regards.