Upgrading to a Nexus 5

You can go to a store to get a micro-SIM card and ask for your account information to be transferred to the new micro-SIM card.  Your smartphone data plan will have to be updated to a "Data Unlimited for 4G LTE Smartphones" plan, but that is just for techincal purposes since you are switching mobile OS.  The features and price will remain the same.

Also, whatever you do — do not switch to a tiered data plan. You can start by just cutting your current SIM to a micro-SIM size, but if you don't start getting LTE speeds you'll need to call or go to a corporate store and get the unlimited LTE data plan instead. Please be advised that with LTE you'll get your data throttled at 5GB/month, 3G plan 3GB/month.

http://www.att.com/esupport/datausage.jsp