How can they change the term if it is not customized? If you have this term not for the PO, then you don't have it for the invoice either if you do not customize it within that time frame.
In the last 39 years with various companies I have seen many initiatives to reduce the used payment terms. This holds good for some time but if your terms finally do not match with those of your vendors then you can't either do this business or one of both parties has to create a new payment term. And so they start to growing again until the next initiative is launched.
If you don't want to maintain it and transport it that often as business requests new terms you still have the option to set the customizing to current and then business can create new terms directly in production system. (you just need to tell them that they must never delete one, otherwise your archiving will fail if POs shall be archived which used such a deleted payment term)