They make you select a plan. If you're on ULD, then you don't want to upgrade, you want to either purchase at full price, or add a new line.
If you add a new line, you have to keep it active for 2 years, but you can keep it active at $9.99 + fees (it comes out to around $16 per month) each month and you don't have to have a phone on the line nor do you have to keep the data on the line. In the end, though, you end up paying more than if you just paid for the entire phone.
Now if you have someone that needs a cellphone, then you can activate a dumb phone on the additional line, but you have to have a dumb phone laying around, or pick one up off Swappa. If you activate a smartphone, then it'll cost you and additional $30 per month for the cheapest 2GB data plan on top of the $9.99 per month.
In the end, you are better off paying full price for the device. I don't know if the 50% off counts towards someone paying full price. Also, I don't know if you can pre-order one at full price. You'd have to give them a call and ask them.