You WILL be able to pre-order.
You will be charged whatever balance remains to reach 50%. For example, if you have paid off 33% of the cost of the phone, you will be charged 27% of the total cost whenever you upgrade to the X. You will then have to return your old device after you receive the X, or pay the remaining 50% to own your old device outright.
You will not be able to order via the Apple Store app however. Apple does not honor device upgrades that have a remaining balance (i.e. not upgrade eligible), you will get an error saying that your only option (via Apple) is to purchase at full price. So you will have to do it through Verizon. This is what I will be doing.