Mismatch between Form 16 and 26AS

I had mistakenly entered my gross salary from Form 16 as nett salary while filing for returns for AY 2012-13. I got a demand notice from CPC for an amount of Rs. 3720. Since this was an error which arose out of the mismatch between my Form 16 and Form 26as, I thought of getting this sorted out and applied for a rectification. I got a message from CPC saying the change of income is not allowed and that changes should have been made under sec.139(5), of the income tax act. My case was transferred to my jurisdictional AO. What should be my further course of action? Should I contact the Assessing Officer and get this sorted out and approximately how much time would it take? I am looking forward to resolving this as soon as possible since I have to apply for a visa in a week or two. Appreciate any help in this matter.