An update on the situation:

Everything's resolved thanks to UA Insider graciously offering to help me with the reservation. She's truly an asset to this forum and it's so refreshing to see people trying so hard to resolve a problem that's really not so easy to fix. So, a big shout out to her.

As far as what I've learned about changing reservations once the outbound is flown:

1) Changing the return portion once the outbound flown is possible. If you don't change the original or destination - changing the res seems to be fairly routine. It can be done online and is more or less painless.

2) That being said, selection is a bit more limited. Some options that are available during the initial booking don't show during the change process. This may lead to higher fares, but in my case it did not. (When I was looking to keep cities the same).

3) Once you try to change the origin or destination, the flights begin to price out one-way. This obviously makes the far much more expensive. (I want to add that UA representatives seem not be aware of this. When I asked before the outbound, they assured me it would be on a round-trip basis).

4) Regular customer support needs to call a special department to get a bookable fare if you need to change cities. This can take upwards of 10-15 minutes. During a particularly busy period it took 40+ minutes to look up one flight. In addition, they need to call this department for every option you want to price out. This is a pretty frustrating procedure.

Bottom line: Change your ticket before you fly if at all possible. Once you fly, changing the return isn't too bad if you keep the city pairs the same. Changing cities after outbound is flown becomes expensive and will leave you paying the one-way fare in the fare difference.

Hope this helps info someone in the future.