[Mods- I searched, but couldn't find any relevant threads; please combine if there are]

I've got a Z fare ticket (ZNC50E) for travel later this month. My travel is:
A-B (overnight at B), then B-C-D, with return D-C-B (overnight at B), B-A.

I am interested in adding on B-X-B on the outbound, which would require moving out the B-C-D by one day (or alternatively just moving the B-C-D out by one day).

I've talked to UA reservations, but because travel is <30 days out, it would require a total re-fare to make any changes. A ticket agent did tell me that changes might be easier (and cheaper) to make after the first coupon has been used.

What's the reality here? I would fully expect to pay the $400 change fee, but what else might I be up against, and what are the risks?

TIA