What testing program should a Technical Architect implement to avoid breaking existing functionality?

The correct choice is Regression Testing. This type of testing is specifically designed to ensure that new code changes do not adversely affect the existing functionality of a system. When enhancements or bug fixes are made, regression testing checks that the previous features continue to operate as expected. By running a suite of tests that were previously conducted, you can identify any new issues that may have been introduced by recent updates, thereby safeguarding existing functionalities.

In contrast, performance testing focuses on measuring the speed, scalability, and stability of a system under load, rather than ensuring the integrity of existing features. Unit testing, while vital for verifying that individual components of code work correctly, does not encompass the larger system integration or overall functionality, which is critical when changes are made. User Acceptance Testing evaluates the system's functionality against user requirements and is typically conducted at the end of the development cycle, making it less effective for ensuring that existing functionality remains intact during incremental changes.

Thus, Regression Testing is the most appropriate choice for a Technical Architect looking to maintain existing functionality while developing new features.