Originally Posted by jsloan

Any change to a ticket is going to carry a similar risk, so it doesn't really matter which course of action you choose. Converting a round-trip to a one-way and refunding the difference definitely requires a re-issue.

If the outbound is still available and you have sufficient miles, you might want to book it, then cancel your entire original itinerary, and then book your new return on a separate one-way ticket.
If the outbound is still available but you lack sufficient miles, the change is likely to be successful whether by calling or online. After you make the change, check your email for a receipt. The ticket number starting with 016 should have changed, and the new ticket number should match what you see in the partner's system (online, were possible, or you can call the partner to verify it). The only exception would be if UA is able to revalidate the ticket instead of reissuing it, in which case the ticket number wouldn't change and downstream systems wouldn't be affected. I think that a date change requires a reissue, but I'm not 100% sure. Despite WineCountryUA's advice, I'm not sure that I'd ask if the agent knew how to do a reissue. I'm not entirely sure that agents make that choice -- I think they just send the ticket off for a resync and UA's systems will pick the right thing. But if the outbound is still available, there's a good chance that this will go smoothly.
If the outbound is no longer available, the agent may refuse to make the change. Online may work or it may fail. If a change is made, the chance of losing the outbound entirely is real. You should take the same steps to follow up -- look for the new ticket number and make sure that the partner site is updated with that number.
If UA flies to your destination, there's substantially less risk, because UA can (and will) put you on their flight if needed to fix the problem.