The code itself looks fine.
However, I think that we should disable this by default for existing repos, since it's a feature I'd categorize as "possibly destructive", and I'm not sure we should spring this kind of features on people with repos before they get a chance to see it. As discussed on IRC, maybe just a small script that sets it to False for all currently existing projects that don't have it set? (or per your suggestion: alembic revision)
Looks good to me.