Can I get a refund on my Sony OEM install of Windows XP Home, considering that I've replaced it with a (licensed) retail copy of Windows XP Pro?

Yes, assuming you never booted XP Home past the license agreement screen. This was actually my situation with Gateway. I wanted to never run Windows 98 on it, and instead wanted Windows 2000. I had a legal copy of 2000 I wan going to use, and when I first got the laptop, I simply tossed in the CD to load it.