Alright, let's clear a few things up.
First, the person in the store was not correct, you can indeed upgrade your phone and keep your unlimited data plan, however you may not like how you have to do it. To keep your unlimited data you'd need to pay full retail price, and not renew your 2 year contract. This phone at full retail price on Verizon's website is going for $499.99.
If you pay the $50 you are purchasing this phone and getting a 2 year contract in which case, you will need to get rid of your unlimited data, you likely pay $29.99 for your unlimited data, minus any corporate discounts, you seem to be on an older nationwide plan, so you could keep your nationwide calling plan instead of going to one of the new "More Everything" plans, and simply choose a tiered data package. They start at $30 for 2GB of data, which sounds like it would be plenty for you. If you receive any corporate discounts though, tiered data plans do not qualify for the discount unless they are priced at $50 or greater. So even though the actual data plans are about the same price, your bill could possibly go up a few dollars.
Finally, the rumor of Verizon taking Unlimited data way is just that, a rumor. Nothing that store employee says makes it anything more than a rumor. Verizon has not announced anything, and store personnel are usually one of the last few in the company to find out about any plan changes, especially in this case seeing as she does not work for Verizon.
And let's put aside the fact that Verizon has told all of its unlimited data subscribers they can keep unlimited data if they pay full retail price, they didn't put an expiration on that offer, so I personally am not concerned about losing my unlimited data plan
Verizon also has a couple data plans for unlimited data subscribers called "Verizon Max" if you go to a corporate store, they may be able to get you on one of those plans and upgrade you at contract renewal pricing, they start at 6 GB for $30, or 8 GB with tethering for $50.