In theory, translating an interface-based monolith to a SOA should be straightforward - drop-in some replacement classes for your services and you're good to go.

In practice, it's not as simple as that. Serialization across service boundaries requires a bit of thought - invoking a method via a local call stack can accidentally cause a blowout on an SOA service buffer. Network timeouts suddenly become a thing. Latency might be an issue.

Of course, these aren't critical obstacles, and I agree that the architecture should look very similar, no matter whether it's a monolith or microservice. But designing one from the ground up would look a bit different.