To the first question, yes, you can port your number. I'd AOL an account first, and then port numbers later. It's super easy to change your mobile number on Verizon, and there is no fee if you do it online. A new number request is instant. A port can take a day or so. Be advised that a number port will cancel your AT&T account, so if you have UDP from AT&T you'll need to give that up in order to port the number.
Regarding the second, if you purchase the "iPhone 6S" at full price you'll preserve your UDP. If you are looking for a subsidized phone, it will depend on whether the "glitch" is still present or makes a re-appearance after VZW inevitably closes the loophole. If you add a subsidized iPhone to an existing line it replaces your UDP with a tiered plan. What people are doing here is adding a new line, which makes the unlimited data plan appear as a choice if you select the Nationwide Voice plans.