You're running into several issues here, but in the end the answer is no, you probably can't accomplish what you want to accomplish.

United has become pretty firm on not helping directly with agency-issued tickets (although occasionally you can work around it). Further, United no longer allows agencies to issue travel certificates (MCOs) for residual value for agency tickets--the residual value is either forfeit or it has to be refunded to the original form of payment if a refund is allowed. As a consolation, United allows agencies to refund nonrefundable tickets for a fee, but that doesn't help you since you don't want a cash refund, and because you've bought up there is a reasonable chance that a) the agency lost control of the ticket, and b) that depending on how the buy-up was accomplished the full value of your buy-up would be lost if you make any voluntary changes/refunds. (Note that this is generally applicable, but some corporate agreements override these rules so it's possible that your company/travel agency may have more flexibility.)

You'll likely still have to go through your corporate travel agency for any assistance, but they'll likely have to call United in order to accomplish much since they likely don't have control of the ticket anymore. The agency is probably going to be pretty grumpy about it, but it's really the best option.