no, you can call back to T-mobile and they can put you back on your grandfathered data plan. I know this is possible because I do it every day. Who ever set up your new phone did not know that you could keep your old plan.
There is a prepaid plan that is 1500 min or texts plus 30MB for $30 / mo. A good deal for people like me who mostly hang out near wifi but once in a while like to get online while waiting in line at the store. Not so good if you don't use at least 300 voice minutes per month, but then the pay-as-you-go rates are decent, with $1.49/day for web access (30MB) on the days you use it. Perhaps not so likely that they will let a postpaid user switch to prepaid, but perhaps if your contract is up they might let you.
I have a grandfathered My Faves plan and had the $25 unlimited data add on for Android. When I upgraded, they told me the $30 4G data add on was required and provisioned it on my line. I use a lot of data so the 200mb for $15 wasn't going to cut it. My house is concrete so WiFi doesn't work too well for me, lol. So far it seems like I'm getting better speeds. Not sure if it's the new phone or the new APN though. I think the $15 is the least expensive smartphone plan available now. The $10 one is for dumbphones and won't support Android. Did you get your phone subsidized? Unfortunately, if you did, you have to keep the data plan for two years or eat an ETF. It's a terrible trap the carriers set for us.