If you use a lot of data, and want to upgrade, then it is worth paying full price. You want to figure out what you use and then look at the current plans to price it out. For myself, it isn't worth it. I use maybe half a gigabyte per month and am on wifi the rest of the time. But it may be a better deal for you to pay full retail.
Just don't lose that unlimited data. You could also use the Verizon Device Payment Plan
This way you don't lose unlimited data. Don't let Verizon say you do, or that its just for tablets. Its not.
You spread out the iPhone cost over 12 months, the first payment and full tax and a $2 finance fee is due on day of purchase. This can only be done at a Verizon Wireless Corporate Store, not at a reseller or chain store like Best Buy, Walmarts etc.
Bring a copy of a monthly invoice with your plan particulars, so you can compare it to the print out from the store. Oh don't let the store tell you that your receipts and agreement is online and read it online. Tell them to print out the whole transaction so you can compare your invoice plan particulars with what is on the new store copy. The only change should be your provision from 3G to 4G and the EMEID/ESN number from your old device should show the new iPhone.
Watch them carefully, they will try and take your unlimited data away. Be very careful.