I feel like a common refactor in a large project might be a complete redefinition of a model layer exposed via a bunch of structs. How does immutability or pat

I feel like a common refactor in a large project might be a complete redefinition of a model layer exposed via a bunch of structs. How does immutability or patter matching save me from runtime errors caused by changing the fields or types inside a struct? Wouldn’t the pattern matching just fail and go down the wrong code path since it was matching for the old pattern? I feel like immutability is great, pattern matching is great, but static typing is also great and there is no reason why it can’t happily live along side the other two.