On greenfield it's worse, because it means from the beginning you have a whole different approach.

On refactoring it's a bit easier, especially since if you choose the right scenario (big new feature), you can just do it that way and then try to teach by example. It's less scary that way, even though there still is initial resistance. Also I think it's better this way, because when you write the tests for the new feature, you can show them side by side with other tests in the codebase. The simplicity of the new tests, and the fewer lines of code but higher coverage, helps sell the deal.