I have an iphone 4 8gb. I want to switch to a Samsung galaxy s2 x. Currently I have a 150 tab with my iphone. But when I trade in my iphone for the Samsung I want to go down to the 90 tab. (I'm going to keep the same plan I have now, $25/month) Currently I have around $100 tab left to pay. What happens to the rest of the tab that was on my old phone? Do I have to pay out the tab in order to get a 90 tab for a new phone. How do I do this?