I have a similar question. I am UA 1K and booked a TATL flight on ual.com on a CO-operated flight with a UA flight #.

I just found out about status match, so I opened a new CO One Pass account, which is in the process of being matched to Platinum level. When that happens, I'll switch from my UA# to my CO # in my reservation.

At check-in, if they are not doing any op-ups, should I switch it back to my UA #? Obviously I have no miles and no mileage history on my brand new CO account. Does it not matter where I credit my flights from now on because everything will be properly combined from both accounts? Thanks.