hey guys, so i'm in the same boat as a lot of people and am trying to upgrade my phone as well as keep my coveted unlimited data plan. This may be a stupid question, but I just wanted to make sure: If I have a smart phone line with unlimited data plan and a second smart phone line that isn't unlimited, can i simply upgrade the limited smart phone line and then later on switch the phone over to my unlimited plan?  Everyone's been saying to use a dumb line so I'm not sure if upgrading a smart phone line that isn't unlimited would work. 
Also, I'm upgrading from an Iphone 4s to a DNA. The iphone unfortunately wasn't 4g, will I lose my unlimited if im upgrading from 3g to 4g?