Impact of gst on closing stock of july 1st

dear expert,

do a registered dealer have to pay the difference amount of tax on the closing stock in order to sell those goods under gst regime.

for example:if the vat on goods is just 5%, but under gst law the tax on such goods is 12% then will the dealer has to pay the diff amount of 7% to the govt. ?

if yes, will the dealer be able to collect the refund on the diff tax paid.