I am traveling this path right now with my son moving to Thailand. The SSI office in the United States will not let you draw SSI in another country, but they will let you draw your SSDI. Just went there yesterday to pay back the SSI they overpaid him.
I had trouble with the system in January trying to fill it out for my son.  I did it four times each time costing $40. I finally just said forget it. He got on the plane and flew to Bangkok and had no problem getting in without it.