I've inherited a lot of poorly designed code; the code has no tests. I am putting tests in place before I attempt a major refactor, but I have run into a problem with my unit tests.
The problem is, I will unit test a function, and then later decide that function is a liability. I will either get rid of the function, or change its purpose. Either way, the original function gone and the unit tests fail.
Should I spend a lot of time putting unit tests into a design I will change dramatically? Wont those unit tests be useless after a major redesign?
Would integration tests be more valuable? I can chose program features I know will not change and create some integration tests to ensure they, indeed, do not change. I might change all the code behind the feature, but the feature should still work, and the integration tests should still pass.
I know there are lot of questions on similar topics. I am asking specifically about the use and value of unit test vs integration tests in messy legacy code.