If you backup a database that, for example, contains null in a field defined as not-null, you cannot restore it. You can use "-no_validity" switch, and you?ll loose _all_ your database checks.
I?m proposing to implement a new switch, that enables to restore a database with checks, even if there are "non-valid data" records.
If you can add a "not null" field to a table, even if is automatically set to null when there are records, you should be able to restore that DB from backup with same state. If not, force to define a default value when adding fields.