I am looking at purchasing the Galaxy S8 plus unlocked from the Samsung website. Am I able to activate the phone by swapping the SIM card from my existing boost phone?
S8 / S8 Plus U models does work with sprint if you bought it unlocked paid SPR.
But you will need a sim card that's compatible with the S8 Plus.
I have been activating S8 / S8 plus OEM purchase phones for few months now but it does require the exact sim that is for S8. You can order these through boost mobile customer service. Once you receive it, go to your local boost mobile store and have them activate it.