When a part is designed, assigned a part number, and manufactured, it is good practice to not change form, fit or function. If someone walks into your warehouse looking for part 123-AB3321, whatever they find should function as that part.
This point has been brought up repeatedly on Eng-Tips. Use the Google window, above, and search on "form fit function".
If the new part is backward-compatible with the old part in all places where the old part was used, then it can be a new revision of the same part number. If not, it needs a new number.
Ticks rule is slightly incomplete from my understanding of what ASME Y14.100 says about when to assign new part numbers. It actually lists several cases when a new PN is required but the most common consideration is are the new and old versions completely interchangeable, both backward & forward compatible.
For example, could both versions be put in a bin on the shop floor and used at random with no problems, including any related to tracability.
'Changes to form fit or function' often get trotted out as the be all and end all of when it's a new rev but even that's incomplete.
KENAT,
Have you reminded yourself of faq731-376 recently, or taken a look at posting policies: