Current UA policy (other than the amount of the fee) is the same as sCO. If you cancel your return, you will have the value of the return to use for a year. When you use that value, you will pay a $200 change fee in new money.

Note that the value of the second segment may not be 50% of the round-trip and, depending on the fare paid, may not even be worth the $200 you will eventually have to pay to use it.

You will earn the miles due to you for the segments you actually fly. But, don't book the new one-way FF ticket until you've cancelled the return. UA's software may pickup conflicting bookings and cancel one or both.

Smart not to rely on SDC. EWR-CLE on a Sunday can be a zoo.

Your best route here depends on what you paid for your ticket and what your tolerance is for the new ticket.