I’ll answer both questions at once 
ADP will apply to all new notes, as well as any existing notes you edit. Attachments are already covered by ADP, so no worries there.
We don’t offer a built-in way to convert all old notes, but it’s easy to do if you want:
Just select your notes and make a small change, like toggling them to read-only and back, or pinning and unpinning, and they’ll sync under ADP.
That said, we don’t recommend doing this for everything, since syncing a lot of notes at once can take time. You can always just update the notes you care about most.
Also, keep in mind: that enabling ADP means you won’t be able to use the web version of Bear.