How do changes in certification requirements affect previous type certificates? How does the 2016 part 23 airworthiness requirements rewrite affect older type certificates? 


If some aspect of an aircraft does not meet the new requirement do all of those aircraft have to be updated accordingly? 
If so, what if the aircraft can't be made to meet new requirements, such as climb rate, etc.
If one of the new rules are less restrictive than the old can those aircraft take advantage of that?
Do newly manufactured aircraft of the same type have to get recertified?
If old type certificates are still valid and the FAA wants one of the new rules to be retroactive will they have to issue airworthiness directives for each type that doesn't meet the new standard?
New rules don't affect t existing designs.... 

Certification basis is declared by the maker and agreed with the certifying authority...Or authorities... ( ideally it's a common certification basis to avoid extra testing, or building aircraft to  different standards) That basis must reflective rules in existence at the time of application. New or pending  rules may be imposed by Special Conditions or equivaent. Any modifications to that approved design are normally to the same certification basis. 

But if a new design comes along, it's the applicable rules at the time of application.. 

If a change is needed to all aircraft of a certain "class" e.g. Introduction of TCAS, ACAS , then that is imposed by operational requirements, such as Parts 91, 121, 135 etc or their equivalents.