3 weeks ago, I purchased a ticket on co.com that had 2 ERJ-145 segments.

At the time of purchase, the exit row was restricted to Platinum Elites. There were other "premium" seats that I was unable to choose.

I put my Continental number in the reservation when I booked it. As far as Continental is concerned, I'm about a sliver above nobody.

I checked in online at the ~20 hour mark. I was then able to select the exit row on both segments.

At the airport, I swapped out my Continental number for my Delta number. No issues/attitude and I retained my exit row seats. My boarding passes now read "Elite", refelecting my Gold status with Delta.