We almost never write software without using some sort of base support. This base support comes in libraries of code developed by other teams that we can just bring in and use. Sometimes we get source code for those libraries. When we do, there's a strong temptation to make changes to that library code. The problem is that the third party teams inevitably come up with new versions of that base code and any changes we made to the previous version likely need to be re-applied to the new version. Making these changes is a difficult problem and often prevents you from upgrading. I always try to find ways to avoid changing the base code. If I absolutely must make changes, then I carefully document those changes. This documentation helps you later identify what those changes were and why they were made. This makes it easier to upgrade to new versions of the base code.
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