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It is precisely when you want to change everything that types are useful, because the compiler will helpfully point you to all cases that you forgot to change.

No matter what stage of the project, you're implicitly writing code to a specific type whether you know it or not. Typescript just makes types explicit, and makes clear all the places you've made type assumptions. So if you need to change types later in the project, you can do so with confidence knowing that you've handled all the cases you made type assumptions.

Some of us only see a value in dynamic typing for shell scripts, burned too many times in large code bases.