Ok you may have to cut a small hole around the coupler and see what is behind the wall more than likely from my experience you will be able to unscrew the existing coupler and screw in a new 1/2" in place that has 1/2" threads on both sides Im betting that yours is 1/2" x 7/16" and then you can convert from there over to shutoff or what need be if its copper then solder on a coupler and extend and put on a compression stop.

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