I have a iPhone 4 with an grandfathered unlimited data plan. I was wondering if I upgraded my phone to an andriod device would I keep the data plan. I have an iPhone 4 with At&t and I am looking to get a Nexus 5.
Google only sells unlocked phone, not plans. You keep all your existing plan. Simply take out the SIM card from your iPhone 4 and put into N5. Call AT&T to have your data plan provisioned to LTE. You are good to go.
As long as you don't sign a new contract you should keep it. "Upgrading" a phone is where people lose it. Buying outright on Google play and activating on your account should keep your same unlimited contract.
Upgrade a phone won't lose it either if you know what to do. Internet sites like Amazon etc. won't automatically let you carry the old plan but you can call AT&T support afterwards to get it back. In most retail stores, you can keep it as without problem if the rep knows what he is doing.
AT&T does not want to provide anyone with unlimited data. Any excuse they can find to get rid of one of those plans they will use. Asking to update an existing account to LTE sounds like a perfect opportunity because you really are changing your service. I hope the other posters are correct, but I suggest you go to a store and speak with someone first before you go out and buy a LTE enabled phone.