Jim,
I went to the Purchases folder and clicked on the link "Download Previous Purchases". It showed the status as already "Downloaded", which makes sense.
I then deleted the song and removed the file from my iTunes Media folder. Back in "Downloaded Previous Purchases", the file now was available for download. I downloaded it and watched the progress bar as it downloaded. But when I checked the song that was now back again in my library, it was still the same 128kbps Protected AAC version as the original.
Which also made sense to me, as just weeks (months?) ago they were charging $0.30 per song to upgrade to the new iTunes Plus 256kbps non-protected version. For whatever reason the iTunes Plus $0.30/song upgrade offer is now gone from the store and has been replaced with the iTunes Match $24.99/year offer.
I could be missing something, but I don't think so. I think the re-download mechanism will only get you what you originally bought, and if the original purchased files were the 128kbps Protected version, that's what you'll get again, not the upgraded iTunes Plus version.
Steve