Yes you can do it. I did an in "Windows Any Time Upgrade" on my Acer laptop to Win 7 Pro. As long as the key is valid it does not matter if it is an Upgrade key or a Full version key and it is x64/x86 respectively (meaning, you can't use a 32 bit key with x64).
To do this, just search for "Windows Any time upgrade" in the start menu.
backing up and saving is always a safe way but i feel you can just upgrade and not have to full install..