Researching a new armor level does not automaticly updates the type of armor for class design.
It will update new designs but not the one that were created before the research was completed.
IE: NameOfClass MK1 has conventional armor -> Duranium armor is researched -> NameOfClass MK1 still has conventional armor, even if not locked, not produced, etc...
NameOfClass MK1 has conventional armor -> Duranium armor is researched -> NameOfClass MK2 (copy of NameOfClass MK1) still has conventional armor
Any design created after the armor change will be correctly impacted
Edit: yeah, there's a big button Update armor....