Does anyone have a good way to distinguish between - A) Rewriting the codebase. B) Fixing & modularizing the codebase - breaking it up into self-contained uni

Does anyone have a good way to distinguish between - A) Rewriting the codebase. B) Fixing & modularizing the codebase - breaking it up into self-contained units/parts while improving, cleaning, and modernizing them? Because if you throw away the edge-case of completely starting from scratch, B) can easily be seen as A) and vice versa. Is there a thin line here? (*I have a 200K line C#/.NET app that will be evolving and pivoting with it's next major version).