I've had international tickets from NW and AA endorsed to UA a number of times in the past. I've had quite different experiences with the two airlines:

NW have agents that absolutely refuse to endorse them. I had to go through a whole song-and-dance and they will just not endorse it whatsoever. He will happily refund the ticket, but not endorse. It takes at least an hour to finally get someone to agree to endorse. You have to wonder why I would want to go to another airline instead of staying with this fine establishment.

AA have no problems with the endorsement and doesn't even care why you want to do it. I actually felt bad about it as they did treat me quite nicely.

It is even more complex when you're at an international location and AA is represented by its OneWorld partner.

I have to add that I've not done this since last September, and the airlines may be even less willing to endorse away revenue tickets.

BTW, those were either full Y or C paper tickets.