6 months GSTR 3B filed with incorrect invoice tax rates, with tax paid and ITC Claimed

One builder told me that his accountant has filed GSTR3B with 8% tax rate (Affordable houses) even after April, 19 for 3 months and has claimed ITC. But the tax rates has changed to 1% with no ITC allowed, so what should I suggest and do in this case, since ITC has been already claimed in all the 3 months.