Generally you have to book the one way in a single class. Needs to be business or economy all the way TPA-IAD-GVA. The only way to book different classes at time of booking would be to cancel the current reservation and book separate PNRs for TPA-IAD (economy) and IAD-GVA (business).

If you want to go the upgrade route you could book in economy and then use the cash upgrade option in the reservation for the IAD-GVA segment (not sure how you can find out the upgrade amount before the fact). Or if there is PZ/PN open on IAD-GVA, you could rebook the entire segment in economy and then upgrade IAD-GVA with PP or miles (assuming you can use PZ/PN).

My advice would be to not risk it and just stick with the early flight. Have some early morning PDBs to compensate