Did you call in or price the change online?

If you just priced it online, I've had cases where the cost to change a ticket online was extremely high, much higher than lowest available fare minus residual value on the return portion plus change fee. Seemed to be an issue of the online change ticket process not pulling up the best available fares.

If that's the case for you, call in, and they should be able to price it out correctly. Unless your ticket has less residual value remaining than the change fee, you should always pay less changing your ticket than booking a new one.

Worst case, note the fare basis on the ticket you want to buy, then call in and tell them you want to cancel your return and apply the residual value to the new ticket.

Unfortunately as this is the "new" UA, you'll need to pay the change fee up-front, and then apply your residual value to the new ticket. But if the residual value exceeds the new ticket cost, you should end up with an ETC for the difference you can apply to a future flight (within a year).