First, and from what others here have said, all "new customer" means is that you have to use a new sim card/set up a new account. Others here have claimed to have had a previous T-Mobile account, some even saying they managed to transfer their old T-Mobile number onto their new account.
Also, to expand on what Atmazzz said, you need to make sure you request the Walmart $30 plan if you have to call to set it up. You need to make sure you are clear, as there are two $30 plans and the other plan does not come with data but 1500 minutes or texts.
In my case, I tried to register online but had a problem, so I had to call. To get me on the Walmart plan, the phone tech set up the plan, then had to get her supervisor to actually put me on the $30 plan (since it is a Walmart or online only). It took a bit more time than I would have liked but I am on that plan, now I'm just waiting for them to transfer my old number from my previous carrier.