If I understand the OP correctly, the ticket is already issued in Business Class. So if to changes the date he must downgrade to coach to confirm a seat, then I think you would get upgraded back to business if space was available since the coupon is for business class.

Conversely, if you get a business class OW ticket issued with a sector in economy (as I did last month), I can verify Number_6's comments that an upgrade is well-nigh impossible.