Upgrade Cell Phone, keep unlimited data:

We have a Droid Bionic that's getting pretty old (for a cell phone) and I was reading a post here in the Technology Forum regarding upgrading hardware.

It's my understanding that I can still purchase a phone for full retail, and transfer my current (unlimited) plan to that phone. Please correct me if I have that wrong.

What does it mean when the phone is advertised as "unlocked"?

I've been reading about simply swapping SIM cards, but I don't know how this works and what it accomplishes.

We use PDAnet (paid version) and would have to have a phone that can run this software.

I'm only interested in a phone with a removable battery, and one that has an HDMI port. It appears that I could purchase a phone from Amazon, Costco, or Verizon. Are there other vendors that are reputable?

One last question, we're planning on traveling to Italy next year, when a phone is listed as "International" does that mean that it would work in Europe?

I'm leaning towards the Samsung Galaxy Note 4, but I will look at all suggestions.

This Bionic has been a great phone, but it's time to upgrade.

Thank you in advance for your time.