How to mass change a property

What is the best practice for mass-changing e.g. 10.000 existing document setting the default vault of a newly introduced property - or e.g. replacing choosing user account property by choosing employee object where the formerly chosen user account is assigned to.

I would open a script console and write a sql like skript for example but how to? A little tool using REST API would be a solution - but perhaps with low code interface like in CK module Managed properties?