How can I use this T-Mobile data plan?

I have ordered the Karma Go wireless hotspot but it doesn't ship until April. Right now I have a pay-as-you-go plan on T-Mobile ($100, doesn't expire for a year, no data). I discovered a plan with unlimited texting and data on their website for $30/month. I am interested in this plan, but I only want to use it until Karma gets here. I just barely refilled my prepaid plan, so I don't want to just ditch $100. Also, this plan is marked as "Legacy" on the website, which means they don't offer it for new phones anymore. Would I be able to use the $30/month data plan for 2 months, then go back to my Legacy plan? Or would I be locked in to the data plan? Could I get a refund for the money on my prepaid account right now? Maybe I should just wait until Karma arrives.

I plan on going to a T-Mobile store sometime in the near future, but I was wondering if anyone would know before I go.