Difference in tax rates under existing vat & gst

i require a small clarification regarding ITC,

At present Goods ( footwear) are under 5% and exempted and others 

we had came to know that under GST, rates of footwear are 0-500 = 5% and above 500 = 18%

So at present what we had purchased (includes in our closing stock contains the above exempted & 5% goods, but under GST regieme, earlier 5 % goods had came under 18%, 

In our presumption we are thinking that ITC will available for what we paid only, so the remaining 13% was loss

please give me a clarity on this, or my presumption is wrong