Making the switch from Boost mobile

Why would T-Mobile allow you to keep your Boost Mobile plan? Virgin Wireless was bought out by Sprint in 2009 and they did offer extremely tweakable "custom" service plans. Boost Mobile was a also a subsidiary of the Sprint Corporation but in July 2020 Boost mobile became owned and operated by Dish Network. T-Mobile did, in April of 2020, become the new owners of Boost Mobile but the merger between Sprint and T-Mobile was slowed by the United States Department of Justice, after it raised concerns over a lack of competition in the telecommunications industry. Eventually, Dish entered the talks and agreed to purchase Sprint assets, including its prepaid business. As for current promotions on T-Mobile though, you can find all the information here. 

Even if Tmobile let you keep your plan when you switched once you buy a new phone from them on a promo you would need to switch to a Magenta plan.

for what its worth you would have known the TMO site here is peer to peer with TMO moderation to keep things in order only...(meaning any answer you get will be from another customer).. if  you would have done your own “know it all fact checking”.