I played through WD on disc on PS3, and now I'm playing through 400 on PS3. It's asking for disc or digital. I no longer have to disc version (thinking about picking up the digital, though). Will be save still copy over to the digital?
Your save will transfer between the disc version and digital version.
In order to play 400 Days you will need either the disc, or at least the digital version of Episode One. Episode One is currently free on the North American PlayStation Store.