They're essentially doing open-heart surgery on a real-time machine with thousands of cogs ad millions of gears. Especially if they're actually cutting over the PSS (which seems like a logical conclusion based on how much won't work) the question isn't will anything go wrong it's how many things will go wrong.

I expect that it will be better than the previous PSS cutover but given the trauma inflicted for me -- who existed essentially entirely in the pmCO world and did a fraction of the travel I do today (I think I struggled to hit Silver that year if I am recalling correctly) if it's 1/10th as bad as then it will be bad in an epic way -- I had flights appearing, disappearing, being double credited, being improperly credited, etc. for weeks... and theoretically, at least, I wasn't even changing systems.