[FungalRaincloud]

I can't think of a case where that's not just a quick find/replace to fix. Or, if you're using a more modern IDE, a refactoring rule.

Do you have something in mind that might be more complicated?

[bluGill]

I write a library that is used by many downstream projects. If I make the struct->class change in my code all my users who have predeclared my class have to fix their code. They probably won't read release notes, and might be using a permissive compiler (gcc is, clang is not - at least in my experience) so it can be a long time before they get weird bugs reports, which take them time to understand.