I'd probably buy a refundable fare and then make changes to that ticket. That should work.

On a side note, I've recently tried to find out some details about MCOs, and I've found NW agents to be pretty clueless (3 calls, 3 different answers). I would really appreciate if some FTers/TAs/NW agents could share some insights. I would be especially interested in (a) learning about the different types of MCO and the associated rules, (b) how to "park" MCOs by buying and refunding tickets, and (c) using MCOs to buy tickets for someone else. Any insight appreciated...