If you have nothing remaining on your tab balance you would pay $350 on a Medium Tab. If you were to take a tab large then you would be required to change your plan to one of the current tab large plans which makes it more expensive over the course of two years. And even on a tab large you'd still pay $225 on the phone up front.
On the other hand if you have any remaining balance on your tab then you pay that off first either at the store, or it can be charged to your next invoice. So the choice is yours when it comes to upgrading, but those are the options available to you. Hope that helps answer your question :)