In a patient with presumed autoimmune encephalitis, would treating them with IVIG before getting a brain biopsy alter the pathological changes in a meaningful way?  

I don't think IVIG would meaningfully alter the results of a brain biopsy. However, a biopsy is frequently less informative in diagnosing autoimmune encephalitis compared to CSF and serum autoantibody testing. It's worth noting that IVIG can trigger aseptic meningitis, so it's essential to exercise ...

I don't think there's any evidence for that.