IMO if a paid expert were subpoenaed against his will, he would only have to testify as to facts he knew (e.g., test results), not as to opinions. I've never seen it happen, so I haven't thought about it much.
I can't imagine Rodriquez [or any hired expert] changing his mind so late in the game and refusing to testify. And, if he refused, I think most lawyers -- maybe even CA's lawyers -- would not call him; one would expect he would not be an effective witness.
But, as a hypothetical, it's an interesting question. I agree with AZlawyer -- I think a reluctant witness can be compelled to testify about matters as to which he has personal knowledge [e.g. the witness to a gang-related crime who fears reprisal]. But I think a witness with specialized knowledge cannot be compelled to formulate and render and testify about an opinion. [Thus, in a personal injury case, if the plaintiff's treating physician hates lawyers and courts, he cannot be compelled to testify about his opinion concerning the cause of the injury, but like any other percipient witness, he can be compelled to testify about the nature and extent of the injury, and the course and cost of treatment.]
But if an expert hired to render an opinion at trial refused to testify, he would likely face a lawsuit for breach of contract, in which the party who hired him would seek not only reimbursment of the fees paid, but damages for having lost the case as a result of the expert's refusal.