[bluGill]

Your zip plus 4 changes. It isn't worth trying to know as it isn't supposed to be constlnt. If you send a lot of mail there is a discount for using it but you have to update everyone's address often (iirt at least 4x per year)

[fingerlocks]

Source? The numbers correspond to the USPS distribution centers and carrier routes. If the numbers are changing that would imply an increase in zip code subdivisions, making each zip code a better address predictor for a given individual.

[bluGill]

https://faq.usps.com/s/article/ZIP-Code-The-Basics

I don't know how to link to the correct question but there is why did my code change.