There were a couple Destroy All Software screencasts on that topic [1].
In a nutshell, you start by writing a lot of high level integration tests, which will serve you as some guarantee you don't screw up. Then you start refactoring, writing fine-grained unit tests along the way. Once you settle with code structure, you might want to start removing/rewriting test cases you wrote initially in case they seem redundant.
I find introducing high level integration tests to be much harder than introducing lower level unit tests as code starts getting modified. In theory, yes, that's the way to go, but in practice, you probably can't get there without significant refactoring.
Comments
There were a couple Destroy All Software screencasts on that topic [1].
In a nutshell, you start by writing a lot of high level integration tests, which will serve you as some guarantee you don't screw up. Then you start refactoring, writing fine-grained unit tests along the way. Once you settle with code structure, you might want to start removing/rewriting test cases you wrote initially in case they seem redundant.
[1] https://www.destroyallsoftware.com/screencasts/catalog/untes... and further parts 2-4. This screencast is done mostly in ruby.
I find introducing high level integration tests to be much harder than introducing lower level unit tests as code starts getting modified. In theory, yes, that's the way to go, but in practice, you probably can't get there without significant refactoring.