Thank you. The chat agent was able to make the changes with no fare add/collect. She said I wasn't able to do it myself because of the KL segments, two different airlines, which doesn't make sense with KL and DL being partners for so long... but I am not accustomed to flying SkyTeam or making changes, so what do I know.

ETA: I also mentioned to her I was not able to save my credit card info in the app - the submit button did absolutely nothing. In the midst of my multiple attempts at trying to make the changes, I thought I'd add the payment method for convenience to no avail.

ETA again: Just for kicks, I thought I'd see if I could manage future changes, but NOPE, it errors out again with a different message.