I'm using Update Unique Keys v1.0.8. It removed all the define statements from my wp-config.php, but did not replace them. The definitions were just gone. The plugin reported that it had successfully updated the keys.
I'd recommend copying into memory the original keys, then perform a validation after the definitions are replaced. If the validation fails to find new keys, replace the old ones and report failure. This way, at least the config would not be left with no keys.
Notice that this is not the failure case in which the plugin cannot alter the wp-config.php file, because it succeeded in removing the old definitions. I speculate (with no evidence) that the key service was down when I ran the update. Please handle this failure case.
Many thanks for this useful plugin!