GST AMendment for 2017-18

No, you canmpt even amend 18-19 now

GSTR 9 and 9c can't be amended once they are filed!

No, for GSTR1 amendment, financial year 2018-19 , 2019-20 can be selected but fy 2017-18 is not showing. So is there any option that we can amend B2C invoices of 2017-18 into B2B?

Generally amendments in GSTR 1 and GSTR3B for previous financial year can be made
1.upto september month return of the following year (or)
2.Annual return filing

which e4ver is earlier.

so in this case for FY 2017-18 Otion for amendment has been given till 31-03-2019 . So now ur not having any option for amendment in Gstr1.So client can claim input on th basis of invoice , and he can give explanation to dept in case of any issue , Tax amount has been paid by supplier but under the B2C.

No you cannot amend any bill pertaining to Fy 2017-18 in 2020.

Note : Portal will also not allow you do so.