Okay, figured it out, no thanks to T-Mobile Tech Support ("We don't support downgrading the OS."
Here's what you need if the AppLoader isn't letting you downgrade: A copy of Desktop Manager 5.0!
I reinstalled a 5.0 build from like a year and a half ago, and that function is built in! When you select the upgrade software option, it shows you ALL copies of the BB OS that are installed on your PC, including ones that would be a downgrade, and lets you choose the one to install!
I'm not sure how recently that functionality was removed from Desktop Manager, so I'm sticking with 5.0 until they bring it back in a newer version!