Why would you want to cancel the primary and go to another carrier, what sort of phone are you wanting???
If you do that, though, you can just cancel the primary and leave the other 2 lines. You will have to designate one of the remaining lines as the new primary. You'll end up saving 9.99 a month, plus whatever features you have on your line, so probably another 29.99 for data feature and if you have anything else you won't be paying for that either.