If somebody had delusional disorder and suffered a psychotic break (w. negative schizophrenia symptoms), could this be diagnosed as schizophrenia?

Not likely: Delusional disorder is an unshakable belief that is not bizarre, meaning it could be possible even if unlikely. Ex: someone convinced that the fbi or the cia is after him/her. Other than the delusion, the person's behavior remains quite normal.A diagnosis of schizophrenia must include delusions with or without bizarre hallucinations, affecting the behavior for a period of 6 months at least.

Possibly: Delusions and "negative symptoms" (i.e., affective flattening, alogia, or avolition) would possibly meet part of the criteria for schizophrenia. But a careful evaluation would be helpful in getting an appropriate diagnosis. There's more to the diagnosis than these symptoms. One's functioning and the timing of the symptoms are also important, as is excluding other possible diagnoses.

Yes: The emergence of schizophrenia is often marked by both positive and negative symptoms although there is increasing recognition of the importance of negative symptoms in both the diagnosis and treatment of schizophrenia. This is not as new of an idea as some people might contend and the idea of "simple schizophrenia" captures this idea. Also, delusions makes schizophrenia more likely. Stay safe!