Well, my first experiment has been (unsuccessfully) completed. Since I had Squeeze installed on my laptop, I edited my sources.list file and replaced all instances of "squeeze" with testing.
Unfortunately, my installation on the laptop was done sans Internet connection (since the install disk doesn't include wireless support, and I was too lazy to take the laptop to the Ethernet connection), and I found that the installer commented out some lines that couldn't be confirmed. The "apt-get update" and "apt-get dist-upgrade" commands seemed to work as expected (in fact, "dist-upgrade" took quite some time), but the upgrade was unsuccessful, nonetheless. I theorize that, in addition to the commented out lines in sources.list, there may have been some that were omitted, as well.
Not really a problem, though. Since it's a relatively bare-bones installation, I'll download a copy of Wheezy and install it fresh...this time with the Internet connection, so the installer will construct the sources.list properly. Once I have everything right (including the wireless drivers), I'll change everything in the sources.list from 'wheezy' to 'testing', and the installation will be a true "rolling release" distribution.
I also still have the installation on my desktop to play with, and that was
connected to the Internet during installation. I might alter my modus operandi a bit, though. I think I'll dist-upgrade to Wheezy, and then
change it to the testing branch. Upgrading from Squeeze directly to testing may have been too large a chunk to swallow at one time.