I’ve created a PHP script for this purpose as I ran into the same issue. John’s instructions of step #4 doesn’t apply, as it expects the ACF field already exists – and just doesn’t have lat/lng.
Here’s the link to the other forum thread:
http://support.advancedcustomfields.com/forums/topic/how-to-update-google-map-latitudelongitude-after-importing-using-address/
