Hi Donuts, as long as it fits into whatever level of tab you have you should be able to switch your plan. If it's a different level of tab plan then you will need to pay off your tab prior to switching plans..
I am on a tab small right now and want to switch to a tab medium for the extra 500 MB. I have tried to switch it myself, but I don't see the extra 500 MB as an add-on. But basically I can keep the phone I'm using now and just switch plans and get the extra 500 MB, right?