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In court, on impeaching expert testimony: Imagine that Einstein is still alive, that he hadn't done anything since he actually died, that he was never famous (so no one in a modern courtroom except for two lawyers knows who he is), and that the world's knowledge of relativity had not changed since he actually died. At someone else's trial, he testifies on relativity as an expert. Answering questions, he explains both his special and general theories in terms a jury understands. Counsel thanks him. Opposing counsel in cross-examination asks Einstein if his theory includes a "cosmological constant". Einstein testifies firmly: "No." Opposing counsel reads to the court from an earlier publication by Einstein that there is a cosmological constant. Einstein seems urgently about to say something but the judge reminds him that he may only answer questions he is asked and no questions are pending at the moment. Opposing counsel has no further questions, counsel who brought Einstein does not seek to ask anything (perhaps because not expecting this turn of events) and the witness is dismissed. Someone searching history would find that Einstein had publicly said that the cosmological constant was the "biggest blunder of his life", but a jury is not allowed to do outside research before issuing its verdict and this is not a nonjury trial. I understand that impeachment frequently works to persuade a trier of fact to disregard impeached testimony; assume such persuasiveness succeeds in this case.

I think this illustrates an inadequacy of the system of determining the state of scholarship through expert testimony, among other reasons because it discourages bringing an expert who might ever have published a statement they would later dispute, and that would exclude most research scholars with long publication histories. Am I legally wrong about this hypothetical Einstein being easily impeachable? Does this vary much by jurisdiction of place?

I'm not opening a debate on reform, just trying to see if I misunderstand the general law.

Your main misunderstanding is that opposing counsel cannot testify. He can ask a question, such as "Did you say ...?", which provides Einstein an opportunity to answer in a way that maintains the credibility of his testimony, but consel cannot just enter his own testimony into the record. In addition, counsel on "his side" has the opportunity to pose appropriate clarifying questions, e.g. could ask "Did you repudiate that idea 88 years ago?", though there is a problem that the attorney may not have the technical expertise to recognize the subject-matter question that they should ask.

Expert testimony does not attempt to determine the state of scholarship in an area, it attempts to resolve a specific factual matter. The two are not the same thing. Einstein would not testify as to what his general and special theories of relativity say, except in a bizarre universe where some person is accused of the crime of denying the theory of relativity. Instead, he might be testifying for the defense, where the defendant is accused of an act that (it turns out) is physically impossible in light of the theory of relativity. If Einstein had been in the habit of posing random mutually-contradictory conjectures on a weekly basis, opposing counsel's question could be relevant to establishing that the witness's testimony is not reliable. Theoretically, counsel could "hint" that the witness is scientifically flighty, so Einstein would have to have a credible explanation for changing his position. It is easy to demonstrate that scientists in general do change their positions as knowledge expands, so Einstein's burden would not be very substantial.