The $30 plan is for new customers only; what that means for you is that you will need to buy a new SIM card and create a T-Mobile Monthly account. The cheapest way to get the new SIM card is to buy the SIM Card Activation Kit from T-Mobile's website
. It costs $0.99 and typically arrives in a week or less. Previously, they only offered a Standard SIM, which is why there are threads about cutting it, but they now also offer a Micro SIM Kit.
When you are ready to activate your new Nexus, you need to first try to activate the SIM online (it is a Walmart and Online only plan). If you have any issues during the activation process, you will get a phone number that you can call to activate the phone -- just make sure they understand you want the "Walmart" plan and not the $30 plan with no data.
Others here have reported that, after activating their phone, they've been able to call in and get their T-Mobile postpaid phone number transferred to T-Mobile prepaid. You might also look into Google Voice to port your current number to. It has the advantage that, if you quit T-Mobile for a few months while in Japan, and because of this you have to get a new cell number, it gives you a way to keep the same number despite the current number on your cell phone.