@Tapari Bahun-- If you want to file the amended return you can do it now, you don't have to wait for anything.

However, I hope you understood what mech meant by,"बांकी कुरा, सही टैक्स फॉर्म भर्नु तिम्रो कर्तब्य हो.... कही सलाह दिन मिल्दैना... मिलाएर बुझनू". Legally you are supposed to be filing 1040NR that doesn't mean that everyone does that. In fact most international students I know file 1040 and get away with it. I have never heard of anyone getting in any kind of trouble because they filed 1040 instead of 1040 NR. I am surprised that the Social Security Office even sent you that letter. If you want to wait and see what the social security office does next then you could do that as well. I am telling what the proper/legal thing to do is. This doesn't mean that you have to do that. I hope you understand what I am saying.

Disclaimer: I am not a tax attorney. Please consult a tax professional to discuss your particular case before completely relying on any information given by me. I have put in a good faith effort to provide you good advice to the best of my knowledge.