I went ahead and bought up to Business class. You're right. It refared me and sent a new receipt showing Z class on my outbound and D on the return. I called Taca and they too confirmed the business class assignment.

Too bad that UAL.com doesn't have seat maps for Taca, and Taca tells me they cannot assign me a seat because the ticket was issued by UA......She said I'll have to get it at check in. So I called UA back and explained that Taca is throwing their hands up and crying monkey because this is a ticket not issued by them and therefore can't do anything with the seat assignments.

The agent put me on hold for about 10 minutes and came back to tell me she was able to talk to an agent (supervisor?) who took care of my seat assignment. I either got an ill-informed agent at Taca, or their whole reservations department is adjusting to this new alliance and customers from other airlines....