Exactly what you've heard. IMO, probably means that they can't lower the price on existing accounts, but they were nice enough to give you a workaround. So pretty much what you would have to do is cancel your existing tablet line, then add it back after March 1.
Kinda easier to do with a tablet since it does have a phone number associated, but it's not relevant since you can't make and receive calls on that number. Plus it would involve a trip to the store for a new SIM card or waiting for one by mail since once you cancel your existing tablet line, the SIM card associated with it will be de-activated, I'm pretty sure you'll need a new one when adding it back.
I actually want to keep the number on my third line, so I'll probably have to port it somewhere else (GV will charge $20), then back to T-Mobile. Again, probably warrants a trip to the store or online order.
I had to use google voice to port my wife's number from Walmart Family Mobile to tmobile. Was very simple to do.