I think u should google samsung maker of 6 & 6s. You can get that answered. Ive switched from verison to straight talk before. Best thing is check out straight talk first!!
If your phone is unlocked and paid for there shouldn't be an issue doing this. The 6s and 6s+ uses the same size sim card. If your switching carriers you'll have to get a sim card from your new carrier. You won't be able to use the one from your old carrier. I hope this helps...
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