I have 8,175 songs in iTunes. 700 of them were purchased on iTunes Music Store.
I just upgraded to iTunes Plus (for the higher quality more than the non-DRM) and took advantage of the limited-time ability to upgrade existing songs.
Unfortunately there were only 23 of them.
UPDATE: Although a normal song takes about 3 seconds to download in my new apartment, many of the iTunes plus upgrades are taking hours and/or timing out. I wonder if they are being served up from a different location that's overwhelmed.