>Of course you can. For example, constraints inside triggers. A simple insert trigger will only apply on records going forward. Or you can just expand the cons

>Of course you can. For example, constraints inside triggers. A simple insert trigger will only apply on records going forward. Or you can just expand the constraint to superset of two, or figure out how the old data needs to be treated by new system and migrate it. There are many right options if you actually care about data integrity that are not as easy as not caring. Doesn't make it good general advice not to care.