IGST AND IC

In case of wrong classification, the department cannot go beyong 2 years i.e., extended period cannot be invoked. Lot of case laws in this regard in case you want to litigate the issue.

If not, then you need to get the BOE's reassessed, pay the differential duty along with IGST so as to avail ITC without any dispute.

Alternatively, a view is also possible that IGST could be claimed based on TR-6 challan however this view to be taken by intimating the department as the department may dispute.