Samsung S8/S8+ use with US carrier & European SIM?

[FrogLick]

I know that unlocked S8+ can be used with either CDMA or GSM carriers, but how about both? If I activate with my CDMA carrier for my US normal/primary service, can I insert a GSM SIM when traveling in Europe and switch easily between them? Is there a setting that allows selecting which to use? I'd hate to have to cancel my primary CDMA carrier to use an overseas SIM & and reestablish service upon return.