If I ever leave Evernote (and they're sorely testing my patience at the moment...) I'd be hauling 52,000 notes behind me. I had one go at letting another app try to import the database when it was 'only' 40K notes which took weeks and failed horribly, so I think I'd just downgrade my account to Basic and make new notes elsewhere. Where I have notes in Evernote that I need to edit or keep close in the future, I'd export those notes and convert them on a case by case basis if and when they're needed...