Is it a good idea to write tests for legacy code?

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Paul asks:

Suppose one had a relatively large program (say 900k SLOC in C#), all commented/documented thoroughly, well organized and working well. The entire code base was written by a single senior developer who no longer with the company. All the code is testable as is and IoC is used throughout—except for some strange reason they did not write any unit tests. Now, your company wants to branch the code and wants unit tests added to detect when changes break the core functionality.