Thanks for the feedback, everyone. It's good to hear that one's profile shouldn't be affected by the upgrade. (Though, I'll take the advice and make a backup, just in case.) I think I'm going to get the Megapack.
BTW, here's the link to the page at atari.com for the Megapack, in case anyone was wondering where to find it. (I found the link in another thread in these forums a few days ago.)
http://www.atari.com/games/test_drive_unlimited_megapack
Depending on where you live, the download may or may not be available. I noticed on a few other games at the same site, that the download wasn't available for me. (I'm thinking of getting Moonbase Commander from the same site for only US$5, too.)