Is it not true that changes in compliance standards (the grandfathering was used for) are higher standards more safety intended + safer aircraft or were they introduced for fun?
In the FAA process, every major design change to be approved is required to be considered for application of "changed product rule", which has the theme that a change to an aircraft approved to an older standard may be required to be approved to a newer, or the most recent standard, if doing so is practical, and would improved safety. Basically, you can't keep modifying an old plane to still be old. But, this, as the certification exercise itself, requires correct application by FAA staff and delegated industry people, or it doesn't work well.