Should a fixed single value enum to a extensible enum be a breaking?

Before:
```
interface xxx {
intervalUnit?: "Days"
}
```

After:
```
interface xxx {
intervalUnit?: abc
}

enum KnownAbc
...

type abc = string
```

Should a fixed single value enum to a extensible enum be a breaking?

No source file, test, or entry point is named; start by locating the code that defines intervalUnit and the repository's breaking-change guidance. Determine whether the Before-to-After type change is considered breaking and document the project's ruling.

Written by the indexing model from the issue text.