Hello SuperflySister,
Boost Mobile is a subsidiary of Sprint, and like Sprint, the Boost Mobile network is CDMA. Currently national carriers here in the United States operate their devices on GSM networks as well as CDMA networks. For consumers, the key difference between CDMA and GSM is that GSM phones keep user data on SIM cards whereas CDMA phones rely on network whitelists in order to verify subscribers. So what does this mean for you?
If you do want to switch carriers after purchasing a phone that is compatible with Boost Mobile's CDMA network, you will have to get the permission of the carrier to have the phone unlocked for use on another network. Note however that even if you are granted permission by the carrier to have your CDMA phone unlocked, that does not mean that your phone will function on all networks. For instance, Sprint phones will not work on Verizon at all even though Verizon also uses a CDMA network. You can take a Sprint phone to AT&T even though AT&T uses a GSM network, but full service is not guaranteed.
The onus will be on you to research carrier phone compatibility if you decide to start with one network and switch to another. That said, I do not recommend trying to take a CDMA phone from one carrier to the next unless your circumstance affords you no other option.
Best of luck.