There is no such automated standard functionality, to my knowledge. In XD02, the users can easily see the documents for this customer using the menu Environment -> List Documents. From that list the users can open every document with double-click and make changes. There is also MASS transaction that allows to make changes in multiple documents.

Automating this process could be a slippery slope. For example, if you already have a contract with the customer with certain payment terms then aren't you legally obligated to adhere to the terms of that contract? Of course, you can negotiate different terms later and then change the customer master, so that new terms apply to the new contracts. But this does not always apply to the existing documents retroactively. That's the exact reason why customer master changes are not automatically propagated to the open documents.

Changes in the customer master are usually not made with such frequency that it would warrant bypassing the careful review of the open documents altogether IMHO. You might want to re-evaluate the requirement.