These are the plans they have. They are what they call SIM only plans if you bring your phone you don't have a tab to pay so overall you pay less. I'm pretty sure you still switch to these plans even if you have tab outstanding with them.
If you bring your own phone you can stay on the same plan and keep your current Tab as long as you want. If you would like to switch to one of the new plans and your Tab is a negative one, a payoff will be needed.