The site wouldn't break with unpopulated fields - it's just that performance would be as bad as before until the fields are populated.

A full-site rebuild would populate the fields, though that took 3 days when we tried it last. I have a script that I run by hand on a production machine, which is a bit ugly.

We *definitely* wouldn't want to do that in a South migration, because that would tie up the push machinery for days.

I figured a gradual population on view was less bad, because at least it would happen for the most viewed pages first.