Ah, yes: you can't change the passphrase for different notes, just one master passphrase for every note.
Maybe, cause of this, I kept only one note encrypted and I forgot the reason...sorry!
It's long time I'd like to share that I have some note encrypted as you can see in the picture inserted.
I don't remember exactly the process to encrypt that: maybe it was an encrypted note in another Android service (ColorNote?...) imported in evernote...
In "colornote", a less developped similar app (you can't share the notes between phone and computers), it's very easy to do it (a click in the menu bar).