I have a Nokia Lumia 520, which costs the same as a Samsung Galaxy Ace 2 x. (150 dollars each. But with the tab, its free) I have only paid about 9 dollars on the tab. Will I be able to switch to a new phone while I still keep my tab balance of 141? But with a different phone.
You can if you want apply the available Tab (in your case 9$) to a new phone (if you remain at the same Tab level).
Or if you open a new line you can get a 2nd Tab of 150$ to get the phone for free (if you're approved for 2 lines).