Just to be sure before actually updating...., if I save my custom ADP file to a different name, download the latest DD update, do the update, change the custom ADP file to match the original ADP file name, and then import it to the newly updated DD, I should have new projections but still have my original custom ADPs, right?

Yep.

Does anyone know what is the source for the ADPs that DD uses?

Antsports.com is the primary one... although I think Dodds is pulling from more than one this year...