Shakespeare applies.
“What's in a name? That which we call a rose by any other name would smell as sweet.” - Romeo and Juliet, Act 2, Scene 2.
Whether we call it X.5 or not won't change how it plays, or how it interoperates with previous materials. Function over nomenclature.
I know of no way to measure "size of change". I care about impact of change.
Since the content of the books will be different, we'll need to have a way to differentiate them in conversation. But whether it "deserves" a X.5 isn't a functional question, to my mind. Calling them 5e and 5.5e, or 5e and OneD&D are the same, for me.