In case of switchover from taxable to exempt transactions or from Regular to composition whether input tax credit is fully restricted?

Yes, In terms of Section 18(4) of the CGST Act, an amount equal to the credit of tax paid on stock held (inputs, semi-finished goods or finished goods) and capital goods (reduced by percentage points) on the day preceding the date of opting for composition/ effecting exempt supplies will have to be paid. The same can be paid by utilization of credit/ cash payments.

(ICAI FAQ PUBLICATION 06-09-2017 -: Availability of Credit in special circumstances: FAQ NO.30)