Class I (Non-interchangeable Change)-
Items in this category will not physically or functionally meet product specifications as a direct replacement item for all applicable applications using the previous revision. Items in this category require new part numbers to be assigned.
Class II (Interchangeable Change)-
Items in this category retain the physical and functional product specifications to act as direct replacement items for all applicable applications using the previous revision.
"Art without engineering is dreaming; Engineering without art is calculating."
Have you read faq731-376 to make the best use of Eng-Tips Forums?
Isn't it something like MIL-T-31000, I tried to find it in the ASME specs a while ago and had trouble, I'd appreciated if anyone can give the ref.
One thing that occasionally gets missed on Class 1 is that the changes must be both forward and backward compatible. i.e. not only must the new rev replace the old but if necessary the old rev can replace the new.