Custom duty

Hi 

We have been importing resistance alloy wire for last many years under a HSN code attracting 5% duty. Now the customs wants to shift this another HSN which will attract 10% duty and also charge retrospect 5 years. Is that legal and also our HSN code clearly states resistance alloy wire under the customs act, they are looking at the composition of the wire putting it under steel and hence different tariff code. 

Pls advise.

Thanks

Manish