Should you choose to do this, the install routine will place all of your existing data and programs into a folder named Windows_Old. You won't be able to run (most) programs from there, but you will be able to retrieve your data,.
I am afraid Scotteq is correct. A custom installation is the only way to move from 32-bit to 64-bit. I wouldn't recommend solely relying on the Windows_Old folder. It's always a good idea to back up to an external hard drive before doing a clean installation.