Since the unlimited data plan is no longer available for smartphones, it's not something you can switch to (even if you have it on another line).
Here's some information about data plans and pricing that should help you make the best decision for your needs:
Here's a very useful tool that will help you estimate your data usage: http://www.att.com/standalone/data-calculator/index.html
Hope this helps!
The iPhone 8, 8 Plus & Apple Watch Series 3 are here!
Find out how to track your iPhone's delivery!
Let us help you upgrade and activate your device!