Oh, yeah, I didn't even think of this -- hopefully digital purchases are transferrable. I can't see any argument why they shouldn't/wouldn't be, if the games will be backwards compatible (any argument that isn't "Sony cash grab", at least).
The statements around backwards compatibility are a massive relief, at least. I really hate it when consoles don't have access to their predecessor's catalogs, and limited 'virtual consoles' like the Switch have really annoy me.
Maybe with the huge improvements to load times we won't need to...