Thanks, @ehutchins ! @tiago.azevedo, I was able to chat with a member of the PPMI study team who informed me:
“…you are correct that cohort is determined at time of initial Clinical consent/enrollment and stays the same for the duration of the participation, regardless of a new research, clinical diagnosis, or consensus dx following initial consent.
…the other 2 additional diagnosis CRFs could change during a ppts time of follow up – the primdiag Primary Research Diagnosis (made by PI) and newclindx Clinical Diagnosis (reported by ppt if a new clinical dx was made outside the research setting).
Hope this helps!