Yes and no. Yes you can switch back. No, You will have to wait a few months before T-Mobile will treat you as a new customer. I believe it's 180 days, and even then some promotions may not apply.
I'm stoked you're looking to come back and we do hope you make the switch! As mentioned above, you can definitely do so, but I can't promise you'll be eligible for promos. You'll also wanna take a look at our Bring your own device setup guide for some helpful tips with your phone.