If you pay off the tab monthly as part of your bill, the tab bonus amount will be steadily reduced and disappear by the time your tab is paid off, meaning you don't have to pay it.

If you choose to pay your tab off early, the remaining tab bonus amount will be charged to your bill along with the remaining tab balance.

Right but if I pay off my tab an switch phones what happens to my tab bonus, am I even aloud to do that?

"and want to switch back to an iPhone."

Do you mean you are going to buy a new phone on tab? Or you already have an iPhone ( like, a back up/ extra) with you and want to use it? 

If you buy a new phone on tab, then they will ask you to pay off tab. And tab bonus will be added onto your next bill as well. ( Tab bonus can be a big amount. Make sure to check it)

Thanks this is helpful