1% sale value treated as exempt

The rule 42 - 1% of sale value of securities to be treated as exempt sales is applicable even for assessees with export sales under LUT? So I will have to reverse proportionate amount of ITC for 17-18? I have already filed GSTR3B returns upto March 2018. Can I reverse it in the return for April 2018 that I have yet to file? Thank you.