CDMA plans only work on the CDMA network. Last I heard, MTS will not transfer CDMA plans to 4G (HSPA or LTE). If you want to get 4G, you will need to get a new 4G plan. CDMA phones are "hardwired" and don't have SIM cards, so it's not possible to trick the network that way.
You can check with MTS to see if they have any special retention deals for people that want to switch from CDMA to 4G.
I would wait until the new phones come out later this month and see what's is available. It might make sense to get a subsidized phone, since the monthly rates will probably be the same.