I create a  screw-on cap for a coffee mug and release it as Revision A.

If the cap leaks, I change the depth of the groove for the seal. After the depth change, the part becomes Revision B.

Now, Revision A and B will both fit the mug and Revision B is backward compatible to Revision A.

If I change the thread to a finer pitch, creating more pressure on the seal, does the part still become Revision B, or would the part require a new part number?

Now, Revision B will no longer fit the same mug as Revision A, so Revision B is not backward compatible to Revision A.

Must all revisions be backward compatible with their previous revision?

Thank you,

Carl