I ran into the same issue with a Latam-operated leg purchased as part of a Delta flight. I was able to get the refund for the original seat selection payment, but now their website won't let me change the seats they auto-assigned me to after they refunded me. The auto-assigned seats won't have enough legroom for my tall husband, and I'm not able to change them. The Latam website keeps erroring out when I go to select seats. Their system seems very buggyy; the flights -- that Delta changed, not me -- are still not showing on the Latam site (Latam shows the old flights). Plus, Latam is showing 2 different return flights on my ticket -- and neither of them is the correct updated date or time!

I was told by a Latam phone rep that they're not allowed to reserve seats over the phone for Delta codeshare flights, so I'm out of luck and will have to ask for whatever seats are available/remaining when I get to the airport. Sigh.