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Uppers do not have the switches on them. Selective fire switches are on the lower. Additionally, non-selective fire lowers have a "shelf" on them. Uppers can be switched from a full auto to semi auto lower, generally speaking.
However, the bolt carrier group on guns designed to fire full auto (and milspec) are different than several commercial manufacturers semi-auto designs. I expect the commercial BCG would work, but would not last particularly long and could be unreliable. Good manufacturers of AR15's and M4geries use the full auto bolt carrier anyway.
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