Did you ever think about modifying pre-defined content types and site columns, but were not sure if it could be done or how to do it.

This document is your answer: Guidance for editing pre-defined content types and site columns

Here is the conclusion copied from this document:

“We recommend that you do not change pre-defined content types in SharePoint Products and Technologies unless there is no other alternative. We recommend that you create new content types that are based on pre-defined content types, and change the new content types instead.”