I think this may clear it up. This is from Verizon's own website at
Share Everything Plan: Unlimited Talk & Text with Shared Data.
I have an unlimited data plan on my account. Do I need to change it?
You're not required to move to Share Everything but if you do, unlimited data will not be retained on your line. As a Verizon Wireless customer you have choices when you upgrade at discounted pricing. You can choose from a standalone data package starting at $30 for 2GB or a Share Everything Plan. If keeping unlimited is important to you, you can choose to upgrade and pay full retail price for the phone.
Look at the bolded text. It appears to say that you can choose a tiered data plan without needing to switch to Share Everything. Being that this is Verizon's own website, I think this is official enough.
So in conclusion, if you want a subsidized upgrade, you
WILL have to give up unlimited data. You
DO NOT have to switch to Share Everything. You can keep the voice plan you currently have, but have to select a tiered data plan starting at $30 for 2GB. If you switch to Share Everything, then this will cause
ALL of the lines in your family share to switch to Share Everything. This only applies to existing customers, as new customers will only have the Share Everything plan available to them.