Sorry for the bump on this, but I wanted to report in on my results. The short version here is yes, you can in fact port your existing prepaid number over from one T-Mobile prepaid to another T-Mobile prepaid account (although, "port" may not be the right word).
Shortly after posting here, I said screw it and pulled the trigger on a couple of discounted $0.99 "Activation Kits" available directly from tmobile.com. Worst case, I just wasted a few bucks to try and save hundreds, so I was OK with that trade off. Now, I'll chalk it up to the holiday, but they took a solid week to show up (yesterday). No big deal since my current plan is good through 12/30, but if you're near the end of your current term, you may want to keep that in mind.
With 7 days left on my plan, I figured today would be perfect to get the ball rolling. This would give me a few days of buffer room in case something didn't go as planned and I needed to port out to Google Voice. So I cracked open the Activation Kit, went to T-Mobile's prepaid activation page, and filled out my info. This presented the first roadblock: my S3's IEMI number was already in the system. When using this number, it wouldn't let me view the $30 plan I was after. Thankfully, a quick Google search pointed me to the protip of using 999999999999999 as your IEMI and that did the trick.
After selecting the plan, I came to the expected roadblock of porting my number. Both the number portability checker and the form I was currently on said my existing T-Mobile number was ineligible. Clearly this was a silly restriction I knew a good CSR would fix for me later, so I ignored this option and let it pick a new number for me. Checked a few more boxes, filled out a few more fields and my new account was setup and ready to go with a new number. Almost there!
The last part of this was to jump on the phone with a CSR and get them to do what the T-Mobile site said I couldn't do. Not that I have much experience with calling T-Mobile in my less than month of service, but my call around 12pm -5GMT on a Sunday got me right through to a rep. My plan here was to only offer up information she needed, but even that may have been paranoid overkill for this. When I was patched in, I simply stated that I wanted to port a number over to my new prepaid account. She then asked who the current carrier was and when I said T-Mobile, she started to sound really confused, but went ahead and looked into it.
This is where the term "porting" being too strong a word comes in. After a minute or so on hold, she comes back and said instead of moving my number around, she was going to change my current account over to the new $30 plan so my current balance of about $4 would carry over and then close out the brand new account. After taking down my callback number and hanging up, I received a series of odd text messages saying my account was changed, that my balance had been updated, that my balance was now low, and that my account had been updated...again. After about 35 minutes, my callback number started to ring and it was the CSR telling me I was all set. As an added bonus, the "device protection" plan was renewed for another month at no cost to me, saving me that extra $7 (which I always keep on hand for a few months while I attempt to brick new devices).
So yes, you can port your number/switch your account to the $30 100 minute/"Unlimited" 5GB 4G service from an existing T-Mobile prepaid account. Yay, frugality!
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