I can confirm that in KS 7.2.1 DriverFactory#changeWebDriver() method is declared public.
On the other hand, the source code shared at https://github.com/katalon-studio/katalon-studio-testing-framework/blob/master/Include/scripts/groovy/com/kms/katalon/core/webui/driver/DriverFactory.java declares the method as private. The disclosed source code is obviously obsolete.