Welcome to Android Central! MetroPCS and Boost use completely different networks (MetroPCS uses T-Mobile's network, which is GSM, while Boost uses Sprint's network, which is CDMA). So their phones are typically not interchangeable. Besides, MVNOs usually have strict criteria for unlocking their phones, since they're cheap for a reason -- you're expected to use it on their network.
If you want to switch your service to MetroPCS, then talk to a MetroPCS agent about it. I'm not sure if it's easy to port a number on a MVNO -- if you can't, then just start a new line on MetroPCS, and settle your Boost bill, pay off that phone (if necessary), and cancel that account.