I had originally used the Cpanel install of 1.2.2. My host doesn't have 1.5 yet, so I downloaded from here, followed instructions, and upgraded on my own.
Worked perfectly. Less than 5 minutes total. I then installed the "Quentin" theme and reactivated ChaitGear's plugins. Flawless.
The minor glitch? The admin section header showed my blog title "munged" over "WordPress", with the parentheses showing but the "View Site" selection missing. And the "Dashboard" page seemed a bit stylistically out-of-whack. It was the work of but a few minutes to figure out that one needed to insert a "break" in the <span> element of the "wphead" div in admin-header.php.
You guys rock! Thanks. Probably the simplest upgrade I've ever dealt with. (Well, the "Quentin" upgrade to fit with 1.5 was super simple too! Thanks for that PikeMurdy!)