But if you define a new type, how is that breaking backward compatibility?

Because that is missing the point.

All the types that are value types in semantics, e.g. Optional, should be proper value types on Valhalla.

Additionally, they should be compatible with existing code that expects them as parameters, fields,.... without being recompiled from source.

If it is a complete new type without backwards compatibility, no one is going to adopt it, other than a few niche cases.